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
Join the Conversation
Ones to watch

In addition to Fortune's 40 Under 40 list, we're also keeping our eye on these 20 innovators, young guns from a wide range of industries and backgrounds.

Market indexes are shown in real time, except for the DJIA, which is delayed by two minutes. All times are ET. Disclaimer Morningstar: © 2014 Morningstar, Inc. All Rights Reserved. Disclaimer The Dow Jones IndexesSM are proprietary to and distributed by Dow Jones & Company, Inc. and have been licensed for use. All content of the Dow Jones IndexesSM © 2014 is proprietary to Dow Jones & Company, Inc. Chicago Mercantile Association. The market data is the property of Chicago Mercantile Exchange Inc. and its licensors. All rights reserved. FactSet Research Systems Inc. 2014. All rights reserved. Most stock quote data provided by BATS.