54. Kevin Systrom, Mike Krieger

Co-founder and CEO; Co-founder and Lead Software Engineer, Instagram

Since Kevin Systrom and Mike Krieger sold Instagram to Facebook in 2012, the app has continued to grow and change.

Late last year, Instagram launched Instagram Direct, a new feature that lets users send private photos, videos, and messages to up to 15 friends at once. Instagram also successfully introduced ads into users' streams without any major revolt.

The app doubled its user base in 2013. Official numbers haven't been released since September, but we can peg Instagram's user base at about 180 million monthly active users.