Age: 26
Occupation: Founder and CEO of Facebook
Previous TIME 100 appearances: 2
As the brain behind the social-media giant, Facebook's 26-year-old CEO, Mark Zuckerberg, had a year filled with high highs and low lows. Facebook itself reached a new milestone — 500 million users and counting — while Zuckerberg seemed to spend the year fending off attacks about his personality and business style fueled by The Social Network, a scathing Hollywoodized account of the site's founding based on Ben Mezrich's book The Accidental Billionaires. Still, as Facebook continues to insinuate itself into every corner of our Internet experience, the company's $2 billion projected revenue for 2010 will outweigh any number of privacy concerns. For his continued innovation of our social experience, TIME even named Zuckerberg its 2010 Person of the Year, making him one of the youngest recipients ever of the title.