In just a few years, Digital Sky Technologies (now DST Global) founder and Russian billionaire Yuri Millner has made a name for himself as a savvy start-up investor. In 2009, Milner invested $200 million in Facebook and has been steadily building his stake ever since. (He also owns a huge stake in discount shopping website Groupon.) Milner, a former physicist, recently built a $100 million mansion in Silicon Valley.
Facebook stake: $5.5 Billion