Mark Zuckerberg: $44.6 billion

Founded Facebook in 2004 while a college student to connect Harvard students. The company went on to become popular globally and listed its shares publicly in 2012, making Zuckerberg, now 32, a multibillionaire. He's managed to make Facebook profitable where rivals like Twitter have lagged, and expanded it with targeted acquisitions. He and his wife have pledged to sell 99 percent of their holdings in Facebook — over 400 million shares, worth about $50 billion — to support philanthropic causes.