Mark Zuckerberg

The youngest member of The Forbes 400 founded addictive social networking site Facebook in 2004 from his Harvard dorm room. Left school for Silicon Valley later that year; scored initial $500,000 investment from PayPal cofounder Peter Thiel. Venture firms swooned. Today Facebook boasts 100 million active users. Estimated annual sales: $300 million. Today college students' guiltiest pleasure; allows users to send messages, publish photos of themselves (often embarrassing). Expanding off-campus; now boasts users from 55,000 different college, high school, business and city networks. Controversy over Facebook Beacon last year; program automatically alerted users' friends of their activities on selected sites, including Ebay and Fandango. Forced to scale back feature 3 weeks after its release. Microsoft bought 1.6% stake for $240 million last October; deal led many to suggest the company is worth $15 billion. Analysts—and even a few Facebook investors—suggest company's value is far lower. Reached settlement with brothers Cameron and Tyler Winklevoss over allegations Zuckerberg stole their idea for Facebook.