Tech's Best Young Entrepreneurs

Joe Green, 24

Company: Causes

Funding: More than $7.3 million from The Founders Fund

While his former Harvard roommate, Mark Zuckerberg, was off running Facebook, Joe Green was busy studying and drumming up online support for the causes he found most compelling. As an organizer, Green's main hurdle was getting do-gooders to raise consciousness about a cause among friends and their social network. So when Facebook made it easier for outside developers to build applications for its site, Green saw the opportunity. "One of the most challenging points of organizing is literally trying to get people to open their address book," Green says. "And here was an open address book." Since Causes launched on May 25, 2007, more than 11 million people have downloaded the application on Facebook, raising more than $2 million dollars.

Advice: "All entrepreneurs should try—from Day One—to build an "economy proof" business," says Green. "Ideally you want to build a business that has diversified revenue models."