Mark Zuckerberg, Facebook
Mark Zuckerberg, 29, is not only the youngest billionaire on this list, but he's also the only one in which a movie on the silver screen was made about the company he started (The Social Network).
The White Plains, N.Y.-born Zuckerberg has turned Facebook from being something college kids used to look at to kill time before their next class into a global advertising behemoth.
The Menlo Park, Calif.-based social network earned 31 cents a share on $2.59 billion in revenue during the fourth quarter, as advertising revenue jumped 76% year over year to $2.34 billion. Perhaps most impressive is the company's mobile initiatives, as it now makes more than half of its advertising revenue from selling ads on smartphones and tablets.
Facebook ended the quarter with 1.23 billion monthly active users (MAUs), and more than 945 million mobile MAUs.
Numbers like that have helped Zuckerberg take the No. 21 spot on the overall list, with an impressive $28.5 billion in estimated net worth.