As a junior at Harvard, Zuckerberg developed software that helped students connect online. It caught on. Today, his site is the No. 7 most-trafficked on the Internet. Facebook's reach has attracted big media players -- including a rumored offer from Viacom for $750 million and speculation that the site was holding out for $2 billion. Zuckerberg insists he's not selling.

Age: 22
Location: Palo Alto, Calif.
Revenue: undisclosed
Employees: 75