Internet companies have been trying desperately for years to court people like Isaac Gonzalez to use their services.

He is 34 years old, married, a father and a well-regarded leader of his neighborhood association in Sacramento. Local government officials and business owners know who he is. When Mr. Gonzalez weighs in on local issues, his neighbors listen.

And Mr. Gonzalez’s opinions and recommendations are exactly what Nextdoor, a San Francisco-based social networking start-up, believes will be the key to expanding its network’s reach and opening a trove of revenue from local businesses and service providers.

“Building community is really, really hard,” Nirav Tolia, chief executive of Nextdoor, said in an interview. “Trust is essential, and it has to be authentic.”