Also offering a note of courage to hopeful entrepreneurs is the fickle taste of Web denizens. A service that is in vogue one year can just as easily be out of style the next.

“There’s always a cycle of what’s popular in Silicon Valley,” said Mr. Shimizu, citing the decline of services like MySpace, Friendster and AOL. “The Facebook experience can be better, and if we can do that, we can open up a new market.”

A primary reason that Facebook grew to become a central hub of the social networking world is its continuous effort to improve the service by adding new features, said Mikolaj Jan Piskorski, a professor at the Harvard Business School who studies social networks.

“When you look back at how little MySpace changed between 2005 and 2007, it’s staggering,” Mr. Piskorski said. “For Facebook to be taken over, there would need to be a drastic slowdown in the rate of innovation. It would take a lot of work to undermine what Facebook has achieved so far.”

Michael Chisari, a developer in Chicago, said the escalating privacy concerns around Facebook spurred him to resurrect Appleseed, an open source project to develop free software that would allow users to set up their own social networking hubs.

“In the past month, there has been a sea change in the number of people looking for alternatives,” he said. “A year ago, nobody was interested in my project, and now I have about 80 supporters signed up.”

Image Leo Shimizu, left, and David Chen founded Pip.io, a social site that emphasizes privacy. Credit... Peter DaSilva for The New York Times

Mr. Chisari, who estimates that Appleseed is six months away from opening to the public, is not the only one trying to create an open alternative. There’s Diaspora, the brainchild of four New York University students who raised more than $180,000 in seed money through Kickstarter, an online site that helps creative people find support. And there are several networks already up and running, like OneSocialWeb, Crabgrass and Elgg, to name a few.