Tumblr

Tumblr

Entrepreneur: David Karp, 22

Funding: $5.25 million from Union Square Ventures, Spark Capital, Betaworks

Somewhere between full-throttled blog software, such as WordPress, and minimalist microblogging tools, such as Twitter, lies Tumblr. New York native David Karp introduced Tumblr in 2007, using money and experience he'd acquired as a software consultant for online parenting site UrbanBaby. Tumblr has attracted 800,000 users, including pop music stars Katy Perry and John Legend, who use the free service to keep an online journal of photos, videos, music, and text. Karp says $4.5 million raised in December gives Tumblr plenty of money to stay focused on attracting users. But he expects the company to generate revenue this year by customizing its platform for big media companies and by charging regular users who want extra services, such as more visibility for their pages.

Lessons learned: "Most of the good decisions I've ever made I can credit to the smart people around me," Karp says. "My role models and my mentors have made a lot smarter decisions than I have."