Founder and CEO, Facebook
Why he matters: Zuckerberg was a 19-year-old Harvard student when he launched a social-networking site called Facebook for the in-the-know college crowd. Three years later Facebook is the sixth most visited site on the Internet, with some 24 million active users and enough clout to turn down a reported billion-dollar buyout offer from Yahoo (and to make us regret our decision to put him on our online-only list of the 10 Who Don't Matter last year).
In September, Facebook opened its doors to anyone with an e-mail address, and in May it announced plans to add free classified ads. It also gave outside developers access to Facebook's underlying code. In a matter of days, one application, iLike, had attracted nearly half a million users.