Nextdoor, a private social network for neighborhoods, has secured $21.6 million in new funding from Greylock Partners. This round closely follows the $18.6 million Nextdoor raised in 2012, meaning it has raised over $40 million to protect homes in less time than it takes to build one.

In addition to the funding, the company announced its 2.0 version and revealed some impressive numbers surrounding its growth.

When VentureBeat spoke with founder and CEO Nirav Tolia in July, Next Door had 3,600 communities on the platform. Today, Nextdoor is active in 8075 neighborhoods in all 50 states and launches in 40 new neighborhoods a day. Tolia connects the rapid adoption to technological and cultural shifts happening across America.

“There are trends enabled by the intersection of social media and local content,” he said in an interview. “People want to come together to create safer neighborhoods, whether it is to track down a lost dog or warn neighbors against suspicious activity. We are helping neighbors help themselves when it comes to crime and safety.”

Nextdoor is an online place for real-world neighbors to connect. People use the site for anything from discussing crime concerns to asking for babysitter recommendations. The overarching goal is to bring communities together. Tolia cited reports from the Pew Data Institute and Harvard Professor Robert Putnam which found that 30% of Americans do not know their neighbors by name, and yet the strength of a community is linked to the overall wellbeing of its inhabitants.

“Unfortunately, in America we have lost touch with our neighborhoods,” Tolia said. “Everyone lives in a neighborhood and wants it to be stronger. Safety is a universal concern.”

The updated platform focuses on crime and safety. Based on user feedback, Nextdoor funneled more resources into security tools such as urgent alerts pushed to mobile devices and integrations with police and fire departments. These features help form virtual neighborhood watches, where residents can collectively keep watch, communicate, and mobilize when necessary.

Another new feature is the ability to connect with nearby neighborhoods. Privacy was, and continues to be, a foundational principal of the network. People won’t post personal information or trust the site unless they believe it is protected. Membership to a Nextdoor neighborhood is only allowed after establishing proof of residence, and members only have access to their specific community. However, certain scenarios call for broader communication. For example, as a resident of the Mission, a string of robberies in Potrero Hill affects me. The redesign allows for expanded sharing in cases like these.

Even though 8,000 communities is an impressive accomplishment, Nextdoor won’t stop until it has a deep presence in neighborhoods around the world. Most of this hefty investment will expand the platforms and create dedicated mobile apps on iOS and Android. It will also fuel international expansion.

Greylock Partners is the lead investor of this round, contributing $15 million of the $21.6 million. Managing director of Greylock David Sze will join the board, and Tolia said this is the largest investment Sze has made in his 13 year career. Benchmark, which led the first round, is contributing along with fellow existing investors DAG Ventures, Shasta Ventures, Allen & Company and Pinnacle Venture. New investors Bezos Expeditions and Google Ventures joined this round as well. Nextdoor is based in San Francisco.

The site provides a trusted and protected online space for neighbors to engage with each other, with the overarching goal of making communities across America safer and stronger.

Nextdoor is a closed network that members can only join after verifying their home address. Once they have gained access, members can communicate with their neighbors on local issues like crime concerns, public spaces, and babysitting recommendations.

The idea for Nextdoor arose after founder Nirav Tolia learned some startling statistics about the state of the American neighborhood. In an interview, he referenced a poll conducted by the Pew Data Institute in 2010 that found that approximately 30% of Americans do not know a single one of their neighbors by name. He also referenced a book published by Harvard Professor Robert Putnam titled Bowling Alone, which examined the clear benefits of strong communities and the threat weak communities pose to the overall wellbeing of their inhabitants. These two discoveries together inspired him to create Nextdoor.

“Unfortunately, in America we have lost touch with our neighborhoods,” Tolia said in an interview with VentureBeat. “A giant part of our identities is where we live, and while there are plenty of social networks to help us connect with people far and wide, I couldn’t find anything to help us connect to people at our front door.”

The task of creating an online community where people feel comfortable posting sensitive and personal information like their childrens’ schools and details about their homes comes with significant challenges. Tolia listed three issues that were crucial to executing his vision successfully: privacy, user acquisition, and utility.

Privacy may be the most important. In order for real community bonds to be forged, people must have complete faith that they are as secure sharing things online as off. Users must provide proof of residence before they can join Nextdoor. No information on the site will show up in search engines or be visible to anyone outside of the network, not even residents of the same city or school district.

The second challenge was how to create a cohesive community online when it does not exist in the real world. How can you invite neighbors to the network if you don’t know their names, much less email addresses? Tolia addressed this by adopting offline engagement techniques like flyers, postcards, and paper invitations using the good old US Postal Service. Nextdoor pays for all the printing and shipping expenses, and residents are prompted to join by an actual piece of mail that explains the Nextdoor concept and provides sign-up instructions.

Finally, Tolia was adamant that the product is a utility, not a forum for gossip, status updates, or posting photos. Nextdoor is a place to discuss crime and safety, exchange local recommendations, express grievances, and create meaningful dialogue between community members.

So far, 3,600 neighborhoods are on Nextdoor. The company spent an entire year in stealth mode testing out the product in hundreds of communities across America to make sure it appealed to people in all parts of the country, in rural and urban areas, and spanning the breadth of the socioeconomic spectrum. Since its launch last October, Nextdoor has experienced explosive growth and now exists in over 3,600 communities. Twenty-two new neighborhoods are activated every day.

“Everyone lives in a neighborhood and wants it to be stronger,” Tolia said. “Safety is a universal concern. I don’t know too many other internet companies that facilitate safer places to live.”

He embarked on this project after founding a sports startup called FanBase that never got off the ground. Tolia decided to switch gears, take the $11 million he had raised in venture capital, and create a company that would positively impact peoples’ everyday lives.

“The more users communicate using online mechanisms, they more likely they are to communicate in person,” Tolia said. “If you use Nextdoor to get to know your neighbors, you are way more likely to walk outside the door and meet them in person. Ultimately, we are trying to get people to meet in the real world.”

This recent round of funding is the first explicitly invested for Nextdoor and was led by Benchmark Capital, where Tolia served as an Entrepreneur-in-Residence, as well as DAG Ventures, Greylock Partners, and Shasta Ventures. It currently has 30 employees and is based in San Francisco. The money will be used to scale the network, which Tolia believes should (and will) be present in every one of the 20,000 communities across the US.

Nextdoor is creating the “neighborhood graph” by combining the best of  Web 2.0 sites like  Yelp and Facebook, with the features of early Web tools such as Craigslist and Yahoo! Groups to reconnect neighborhoods and re-establish a sense of community, on and offline.

Nextdoor has the look and feel of Facebook, with an activity feed, and tabbed navigation, where you’re able to look for a babysitter, and check out recommendations left by your neighbors. Anyone who has used the Internet during the last decade will feel right at home.

Nextdoor has gone to great lengths to create an experience that is authentic, and where trust and identity are the operative words, says co-founder and chief executive officer Nirav Tolia.

There are four mechanisms for verifying your address, all of which inject significant friction into the sign-up process, he says. You can request postcards with a unique code; your home phone number attached to your home address can be called with a code; you can can enter your credit card for verification; or you can be invited by a verified neighbor.

“The amount of friction is directly correlated to the value you derive from being in an environment is where everyone is verified,” says Tolia.

Of course there’s a business model somewhere in all of this. It could be merchant services, it could be deals, or something else. Though he wouldn’t specify, he said there are lots of potential opportunities around connecting businesses with customers who live and shop in the area, once they’ve nailed the product, and have grown the user base. ”As we’ve done with everything else, we’ll follow what the users ask for,” says Tolia.

It cost the company $60,000 to buy the domain name, said Tolia, but he thinks it’s definitely worth it. He pointed out that all the most powerful tech companies have two “o’s” in the name. Facebook, Yahoo! Google, — even Microsoft. “And now Nextdoor,” says Tolia.

Nextdoor is backed by Benchmark Capital and Shasta Ventures.

BlockChalk

BlockChalk, first covered back in January,  is a location-based mobile service available to iPhone, Palm and Android devices that lets users anonymously leave messages in different areas of a community or neighborhood. These messages can be anything from reporting crime to confessing your love for a neighbor. According to the company website, BlockChalk is in over 128 countries, 10,514 cities and 16,652 neighborhoods.

While it might seem that leaving anonymous messages in your neighborhood could get out of control, the company has set up a list of guidelines. Some of these guidelines including keeping your messages locally appropriate and be respectful of others opinions.  BlockChalk also reserves the right to ask you to leave (just the application, not your actual neighborhood). Things that will get you booted include spamming, criminal activities or porn.

A number of investors joined the first round of funding, include Battery Ventures, Founder Collective, Harrison Metal Capital, Delicious founder Joshua Schachter, Lotus founer Mitch Kapor, Josh Stylman, Tom McInerney and David Liu.

The San Jose-based company was created by Stephen Hood, the former product team lead for Delicious, and Dave Baggeroer of Stanford’s Institute of Design.

Competitors include EveryBlock, a startup purchased by MSNBC.com, and Facebook application Neighorhood.