Pic: Phillip Toledano

As a 23-year-old math genius one year out of Harvard, Jeff Hammerbacher arrived at Facebook when the company was still in its infancy. This was in April 2006, and Mark Zuckerberg gave Hammerbacher—one of Facebook's first 100 employees—the lofty title of research scientist and put him to work analyzing how people used the social networking service. Specifically, he was given the assignment of uncovering why Facebook took off at some universities and flopped at others. The company also wanted to track differences in behavior between high-school-age kids and older, drunker college students. "I was there to answer these high-level questions, and they really didn't have any tools to do that yet," he says.

 After a couple years at Facebook, Hammerbacher grew restless. He figured that much of the groundbreaking computer science had been done. Something else gnawed at him. Hammerbacher looked around Silicon Valley at companies like his own, Google (GOOG), and Twitter, and saw his peers wasting their talents. "The best minds of my generation are thinking about how to make people click ads," he says. "That sucks."