3. Foursquare - Five mobile social networks you need to follow

Introduced in March 2009, Foursquare is now synonymous with the fast-growing location-based mobile social networking segment. Essentially a videogame translated from the virtual realm to the real world, Foursquare awards users points, badges and "mayor" status for frequenting local nightspots and restaurants, also sharing their whereabouts with friends via platforms like Facebook and Twitter. Research firm RJMetrics calculates that Foursquare surpassed the 2 million user count in early July--the startup recently said it's adding new users at a rate of about 10,000 a day across device platforms including iPhone, Android, BlackBerry, webOS and Windows Mobile. In late June, Foursquare closed a Series B venture financing round valued at $20 million.

Earlier this year, rumors indicated Foursquare was in serious acquisition talks with both Facebook and Yahoo, but for now the startup remains independent. Unlike rival social networks that have struggled to identify and formulate a workable revenue model, Foursquare's value proposition is crystal clear--its future lies in location-specific advertising, and the firm already boasts marketing partnerships with brands including Starbucks, Zagat's and The New York Times Company.

Foursquare's biggest challenge is its own momentum: The service is growing so fast that the company is struggling to keep up, and advertising agencies say it's increasingly difficult to get execs to respond to partnership requests. Foursquare admits it has struggled to scale its business, but Ben Horowitz--co-founder of the Silicon Valley venture capital firm Andreessen Horowitz, which led the recent funding round--remains convinced of the startup's long-term viability. "This is a model that users and businesses love," Horowitz told The Wall Street Journal. "It will monetize better than any social networking product out there."