
At 25, Zuckerberg is by far the youngest member of the Forbes 400 list. The next youngest person on the list is hedge fund operator John Arnold, who is 35. Last year, Zuckerberg debuted on the list at #321. His net worth at the time was $1.5 billion.
Zuckerberg started Facebook in his Harvard dorm room in 2004, and now Facebook is the third largest site on the internet. In May, Russian investment firm Digital Sky Technologies invested $200 million into Facebook, setting its $10 billion valuation.
Earlier this month, Facebook announced that they now have 300 million users and cash flow positive for the first time last quarter.
http://www.techcrunch.com/2009/09/30/mark-zuckerberg-net-worth-2-billion/