28. Dennis Crowley & Rio Caraeff (tie)
Dennis Crowley & Rio Caraeff (tie)
Dennis Crowley (l) and Rio Caraeff
Crowley: Co-founder and CEO, Foursquare
Age: 35
Industry: Technology

Many thought Facebook would doom Crowley's location-based software, but in the past year he's grown it from 3.7 million to more than 10 million users, signed up a half-million merchants, partnered with American Express -- and added President Obama as a user.

Caraeff: President and CEO, Vevo
Age: 36
Industry: Music

This year the music video site he runs has gone mobile and produced original programming; it also hit 800 million monthly views, nearly double from a year ago. It doesn't hurt that MTV, once Vevo's biggest threat, has, Caraeff says, "moved on to The Jersey Shore."

Last updated October 25 2011: 5:11 PM ET