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Nextdoor, a social network for neighbors, is moving into elite territory.

The San Francisco start-up announced a $60 million investment on Tuesday, led by Kleiner Perkins Caufield & Byers and Tiger Global Management, two prominent venture capital firms. Comcast Ventures, Benchmark, Greylock Partners and Shasta Ventures also participated in the fund-raising round.

With the investment, Nextdoor has raised more than $100 million in the last 18 months, and that is bound to set off questions about whether venture capitalists are inflating another technology bubble by throwing money at unprofitable start-ups.

Nextdoor has no revenue yet, but believes it is well positioned to capture some of the multibillion-dollar local advertising market. Moreover, the company has used little of its cash pile. With this round, Nextdoor will have more than $90 million in the bank to expand in the United States and abroad.

The company did not disclose its valuation after the new fund-raising round, but its chief executive, Nirav Tolia, said employees and the founding team still had more than 50 percent of equity in the company.

Nonetheless, Nextdoor is now worth at least a few hundred million dollars, making it the latest example of fashionable and fast-growing start-ups commanding startling valuations despite the lack of a proven business model. So far this year, the photo-sharing site Pinterest was valued at $3.8 billion, the unprofitable online retailer Fab.com was valued at $1 billion, and the photo messaging site Snapchat is reported to be seeking a $3.5 billion valuation.

Mr. Tolia said the fund-raising round was more about partnering with talented venture capitalists than it was about filling the company’s coffers. John Doerr, a Kleiner partner leading the investment, famously advised Google as it scaled, while another Kleiner partner, Mary Meeker, was one of the best web analysts of her generation. Tiger Global, meanwhile, is skilled at international expansions.

“Much more than the money, we’re trying to bring in the right people to get to the next level,” Mr. Tolia said in an interview, defending the company’s valuation despite its lack of revenue. “If we didn’t have strong underlying metrics, we probably wouldn’t have raised $100 million in the last 18 months.”

Venture capital firms often pile into well-established start-ups with large late-stage funding rounds as a way to hedge their bets on smaller, unproven ventures. “We didn’t go out looking for money,” Mr. Tolia said. “To some extent the guys who were interested wouldn’t take no for an answer.”

It was only in February that Nextdoor raised $21.6 million from investors including Greylock, an early investor in LinkedIn and Facebook.

“The entrepreneurs that are building really great companies are raising lots of money before they’re cash-flow positive,” Mr. Doerr said in an interview. “Funding brings in strategic options.”

Kleiner’s investment is coming from its $1 billion Digital Growth Fund, which already has late stage investments in Twitter, Spotify and Square.

Nextdoor does not disclose details about its user base, but Mr. Tolia said the site had fewer than 10 million active users. That growing community is highly dispersed across more than 22,000 neighborhoods around the United States.

Despite Nextdoor’s lack of revenue, there is reason to believe the company may succeed.

Mr. Tolia pointed to Yelp, the customer reviews site, and Zillow, the real estate site, as examples of companies that have successfully tapped the local ad market. Both grew popular as unprofitable start-ups, but eventually began to generate meaningful revenue. Since going public in recent years, both companies have more than doubled in value, with Yelp now worth $4.4 billion and Zillow worth $2.9 billion.

And Mr. Tolia says Nextdoor is adding members at a healthy clip and believes it can fill the information void left by the erosion of local newspaper readership and local television viewing. The company is also attracting classified ads, robust discussions on its message boards about everything from crime to local businesses and services, and becoming a forum for neighbors to meet one another.

“We now know fewer of our neighbors than ever before as we are immersed in screens,” said Mr. Doerr, who added that he had recently had in-person meetings with neighbors he connected with on Nextdoor.