By Nancy Scola.

Having her bike yanked from the utility closet of her San Francisco apartment building reminded Sarah Leary why she had spent the last three years building an online social network for neighbors.

“I put out a message saying, ‘Here’s what my bike looks like,’” says Leary, co-creator of Nextdoor, “and I had three or four people chime in with just, like, ‘I’m so sorry that that happened.’” (She added that those few posted words alone “made her feel known and loved and supported.”) Other neighbors were more practical. One insisted that she file a report so that the police might add it to the stats used to track local bicycle thievery.

Oh right, Leary, a Massachusetts native and tech world veteran who now lives in the Lower Pacific Heights neighborhood, recalls thinking. I’m supposed to do something about this for my neighborhood in real life, not just gripe about it online.

The site co-founded by Leary is a simple enough idea. We’ve become acclimated to using Facebook to connect with friends and family. LinkedIn for work. Twitter for our interests. Yet in 2014 there is no go-to online social network for the people we live among. “And that,” Leary says while sitting in Nextdoor’s suite of offices, “is kind of crazy.”

The Nextdoor team, Leary says, draws some of its inspiration from Harvard political scientist Robert Putnam, who concluded in his 2000 book Bowling Alone that “social networks in a neighborhood” make crime go down and test scores go up. Even more fundamentally, our neighbors would be the first to dig us out from the rubble after an earthquake. But today, we don’t know them that well. Nearly a third of Americans can’t pick out a single person in their neighborhood by name. For all the talk about technology driving us ever further into our personal bubbles — Putnam used “social networks” in the pre-Facebook sense — Nextdoor’s gamble is that the Internet can, in fact, be the missing bridge between us and the people with whom we share a spot on the map.

If you’ve never heard of Nextdoor, its team is okay with that for now. Since launching its first network 30 miles southeast of San Francisco in Menlo Park in 2011, Nextdoor has grown to 31,000 neighborhoods across the country where people talk about everything from break-ins to favorite eateries to plans for a new dog park. That said, “we’re not trying to do it overnight,” Leary says. “We’re trying to do it right.”

But in a world of affinity networks, can a proximity network scale?

The team has spent the last few years trying to figure that out. The nut they want to crack is a tough one: Creating a social network that spans the U.S., at least, but one that, when you look closely at, is made up of thousands and thousands of private, residents-only loops. Address is verified by postcard, credit card, phone number, the last four digits of your social security card or by approval of a neighborhood lead. (More on “leads” below.) Maintaining the level of privacy necessary to get users to share the details of their on-the-ground lives with people they don’t yet know is what Leary calls Nextdoor’s “huge burden to carry.”

One solution has been to roll out a system of ‘adjacent neighborhoods.’ Let’s say you live in Neighborhood Zero. That’s your network, and in most cases what you share stays within that circle. But you can choose to share some posts to the neighborhoods next door — helpful if, say, your dog runs away. When your post travels beyond Neighborhood Zero, your profile shifts a bit. By default, your full address (Sue Smith, 26 Franklin St., Chapel Hill) is displayed alongside your name. But outside the boundaries of your neighborhood, only your street name (Sue Smith, Franklin Street, Chapel Hill) pops up. Figuring out the particulars of those sharing settings took a year.

And the tweaks continue. Nextdoor is testing a feature where users in New York City have only their street name displayed by default. The thinking is that within the five boroughs, the level of familiarity you want with someone one building over is about the same that other Americans have with people one town over. But the challenge with such careful privacy-by-design is that it can make it difficult for Nextdoor to spread.

If social networks are a virus, Nextdoor has large gaps between hosts. Its networks start with one founding member. He or she draws neighborhood boundaries using built-in mapping tools that suggest networks amounting to between 50 and 3,000 households. (Leary insists in the face of my skepticism that there’s been little tension about where neighborhoods begin and end.) If you’re one of the first to join that network, few people are there to greet you, even if you have hundreds of friends in their own networks scattered across the country.

Nextdoor grows, then, through what we might call shoe-leather virality. The company prods locals to go recruit new members and help where they can, like through custom door hangers that can be easily downloaded from the site. Right now, it is offering a $25 gift card to anyone who recruits a friend to start their own neighborhood network and adds 10 neighbors in 21 days.

Nextdoor may be harder to grow than other social networks, but there is, it seems, an upside to the idea of locals doing it by hand. It can make the network feel homegrown, and the fleet of software engineers working in San Francisco fades far into the background. Leary tells the story of a man in upstate New York whose wife was a neighborhood lead for their Nextdoor site. At a dinner party one night, his companions went on and on about what technical mastery his wife had shown in “building that network for our town.” Nextdoor tries to foster that sense of local ownership. Thus the concluding line on its manifesto: “We are simply you and your neighbors, together.”

Leary says another lesson is that the checks and balances needed on a network of neighbors aren’t what they might be on other social networks. “Flagging” abusive or offensive content is a standard-issue feature on social sites, including Epinions, the now-shuttered online review community where both Leary and Nextdoor co-founder and CEO Nirav Tolia once worked. When Nextdoor was on the cusp of rolling out its first test network, in Menlo Park, Leary pushed hard to get flagging built into the prototype.

Leary jokes that co-founder Janakiraman would send her weekly email updates: “35 weeks and zero people have used the flagging feature.” There are, instead, gentler nudges toward camaraderie throughout the site. There’s a “Welcome” button that does nothing but greet a newbie, the digital equivalent of “Howdy, neighbor.” Instead of Facebook’s “Like” button, the primary one-click sign of approval is “Thank,” a reminder that polite conventions are expected.

Nextdoor co-founder Sarah Leary.

Team Nextdoor largely leaves moderation to community leads — a network’s founding members, generally, though additional leads can be added. Self-regulating online communities are, of course, always easier on headquarters. But here, where offline communities are recreated online, the model makes particular sense. Leary remembers the appearance, in the early days, of a mildly rude but not hugely offensive comment on the second network launched. She and the team brooded over what to do, and then headed off to lunch for further contemplation. When they got back, there was an email from the local lead waiting for them.

“She was like,” Leary recalls, laughing and waving a finger, “’I just want to let you know, I saw Steve at the coffee shop, and I told him what I thought about what he did.’” There was another email, this one from Steve himself, begging the folks at Nextdoor to delete the post.

Other design choices aim to keep discussion on Nextdoor a tad more elevated than elsewhere online. The site now offers structured topics for conversation — including “Classifieds,” “Crime and Safety” and “Recommendations” — built around what the site’s first users were discussing. Not only is there no “Political Rant” option, but the subjects encourage focus on concrete exchanges reinforced by propinquity. “Are you,” Leary asks, “going to sell a lemon of a car to your neighbors?” They also created a new “groups” option so that people can have especially sensitive or esoteric conversations off to the side.

But Nextdoor, Leary says, benefits from the fact that we, the online public, are more comfortable than ever with the idea that our online and offline selves are one and the same, an understanding fortified by the fact that Facebook’s insistence on “real identity” seems to have won the Internet in just the last few years.

Nextdoor argues that the combination of real names and addresses gives its network a unique power. On Twitter, I stumbled upon a user named @JustAnotherMo commenting that, “I might not be ‘happy’ about how this thingy works, but OMG, the opportunity to shame neighbors gets results.” I press for details, and “Mo” tweeted in response that posts about a neighbor’s front yard “toilet art” got the yard cleaned up within half a day. “In a neighborhood with a lazy [homeowner association], druggies and other assorted problems,” Mo wrote, “ is great way to clean it up.” (Mo declined to email, so I don’t know his real name – or even that he’s for sure a he – but his neighbors on Nextdoor do.) At the time, though, Mo said that he’d stop going on Nextdoor after the exchange.

That’s another challenge, because Nextdoor isn’t exactly, well, fun. It isn’t a place for sharing vacation photos. As a result, Nextdoor isn’t, Leary concedes, an “amuse me” app. “You’re talking with these people because you share space with them,” she says. Interested as you may be in knowing about shifty characters going door-to-door in your part of town, it’s not what you obsessively click on while waiting at the airport. “I don’t want us to be known as just the neighborhood watch tool,” says Kelsey Grady, Nextdoor’s communications lead. Indeed, one in five posts mentions crime or safety, but that leaves 80 percent of the site about something else. (See the breakdown at right.)

The ultimate goal is to make information shared on Nextdoor so valuable that people don’t want to miss it — a bet on the notion that far more people care about, say, what happens at community meetings than attendance numbers suggest. It’s a vision of Nextdoor less as a social network than as a social utility.

Through its Nextdoor City Program, the site is working with about a hundred U.S. cities and investing heavily, Leary says, in figuring out how to best support cities as it seeks to grow and inform networks sprouting up within its borders. When, for example, New York City joined up with Nextdoor in June, it asked for a way to narrowcast updates to the site. As Leary puts it, “if a water main goes out at 20th [Street] and Park [Avenue in Manhattan], people in Brooklyn do not care.” New tools let cities pin their posts to locations and then control how far out from there they radiate.

But cities getting so involved in the workings of an otherwise grassroots site can be unsettling, especially as Nextdoor partners with police departments in cities from San Jose to Dallas to Charlotte. Elsewhere in California, Oakland’s challenged police department is soon rolling out a major presence on Nextdoor. Marrying police departments and Nextdoor makes a certain sense — they’re both a city function natively organized around the idea of neighborhoods. Cash-strapped departments see the appeal of having officers, who might not otherwise live in the communities they patrol, be validated for those networks so that they might easily carry out their purpose as neighborhood liaisons. But there’s the obvious risk of a chilling effect when cops show up on your social network. So Nextdoor restricts cities to only see conversations around what they post to neighborhood networks. All other conversations stay private.

The balance that Nextdoor hopes to strike can be thrilling to those in city government. Michael Powers, legislative director for the president of the St. Louis Board of Alderman, reached out to Nextdoor last spring, and the number of St. Louisans on the site has grown from about 800 in two or three neighborhoods to 10,000 in 68 neighborhoods. As city lead, Powers has the power to monitor what other city officials post on the site. He points with pride to a notice of a theretofore-unknown-to-him tee-ball session. He can also get overview numbers on the activity of each neighborhood network. If limits on the city’s interactions make Nextdoor less burdensome than Facebook — for one thing, Powers points out, Missouri’s Sunshine Law only applies to the city’s more circumscribed engagements on the site — the tone of the site has turned out to produce a far less snarky experience than Twitter.

Powers says he is well aware that as pared-down and simple as Nextdoor’s interface is, not everyone is ready or able to conduct their civic lives online. But, “frankly, we’re okay with that,” he says. City government isn’t going to stop hosting in-person meetings. More than that, though, the city has to start tapping a new reservoir of people who might be ‘community leaders’ but for the fact that they can’t or won’t turn up for a town meeting at 2pm on a Tuesday.

And if Nextdoor upsets the careful balance that makes the site comfortable and fruitful for both cities and residents, Powers says, St. Louis will stop working with it. But he’s betting that they’ll keep getting it right.

Nextdoor hasn’t made any money yet, but it has attracted about$100 million in backing from the likes of noted venture capitalist John Doerr and other tech world figures with experience growing online social networks. (When I, embarrassingly, gasp at the number, Grady says that “a lot of people believe in this opportunity.”) Part of Nextdoor’s attraction is that it has few, if any, obvious competitors. One is Yahoo! Groups, the online discussion boards that Leary calls “a piece of technology that hasn’t been touched in about 16 years.” That level of funding means the Nextdoor team has the time, as Leary puts it, “to get the flywheel going.”

If they can, though, they’ll have a direct line into thousands of what Powers calls “microeconomies.” Leary sees moneymaking potential in letting businesses interact with residents of their communities. In the aggregate, that can mean big business. But even bigger things are possible. Every interaction on Nextdoor is tied to a point on a map — one of the good things, says Leary, about the fact that co-founder Prakash Janakiraman helped engineer Google Maps — and residents in every active house in its networks has been hand-verified, producing perhaps the richest digital social-geography of the U.S. ever to exist under one roof.

But first, more people need to join Nextdoor. The team sees its potential market as massive. Of American adults, college kids are probably out of their reach. That’s okay, because beyond college kids, as communications lead Grady points out, “everyone lives in a neighborhood.” And they believe that the more you settle into a neighborhood, the more likely you are to see the appeal of Nextdoor. That makes it a social network you age into, but there are other ways that Leary sees momentum on their side.

“We didn’t invent neighborliness,” she says. “All we’re doing is creating a modern-day platform that allows people to act on their good instincts.”

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Originally posted at Next City.

The Shared City is made possible with the support of The Knight Foundation.