4. Facebook: Mark Zuckerberg
Mark Zuckerberg, whose storied startup was chronicled in the 2010 movie The Social Network, dropped out of Harvard University in 2004 with cofounders Dustin Moskovitz, Eduardo Saverin and Chris Huges to work on Facebook fulltime.
Eight years and plenty of drama later, Facebook has turned into the biggest social network, with more than 900 million active users worldwide. As its IPO approaches, Zuckerberg could be worth up to $28 billion.