In 2007, when others his age were studying for midterms and living on dorm food, David Karp was busy launching Tumblr, an easy-to-use blogging platform that now hosts 17.5 million blogs and receives about 1.5 billion page views per week. The company has also attracted some $40 million in venture funding. Karp, 24, is used to doing his own thing. At 11, he taught himself how to write code. At 15, h