Net worth: $40 Million (est.)
New York City's brightest high school dropout flipped his earnings from working as a software consultant at UrbanBaby to fund his microblogging empire, which accounts for over 64.7 million blogs and 27.5 billion total posts. Karp's creation continues to serve up some of the Web's most creative and hilarious sites. Take a look at the 100 Best Tumblrs of All Time and see for yourself. Last we heard Tumblr’s appraisal value is estimated at over $800 million.