World’s youngest billionaire leading his addictive social networking site Facebook to an inevitable public pay-day. Firm became “cash-flow positive” last year, instituted dual-class voting structure akin to what Google put into place before it held its 2004 offering. Fresh-faced entrepreneur launched Facebook from Harvard dorm room in 2004. Left school for Silicon Valley later that year; bagged initial $500,000 investment from PayPal cofounder Peter Thiel. Last May Russian investment firm Digital Sky agreed to buy stock from Facebook employees; deal priced company at $10 billion. Shares continue to change hands on private equity exchange SecondMarket; recent transactions put the value at upwards of $15 billion. Growing rapidly: in past 12 months user base has surged 130% to 400 million.