“When I first met David he was 20 years old and wearing sneakers and jeans,” said Bijan Sabet, a general partner at Spark Capital, who was one of the first people to invest in Tumblr. “But I knew he was one of these rare entrepreneurs that grew up on the Web and who could come up with an idea, build it himself, and then ship it that night.”

Since founding Tumblr six years ago, Mr. Karp has been admired for his programming skills and Web site design acumen but at times has been a polarizing figure in New York tech circles because he so often blogged about his personal life and party-hopping. He has popped up in the New York Post’s Page Six Magazine, and has been a recurring target for the gossip Web site Gawker, where he was labeled a “fameball,” a derogatory term for someone who has an unquenchable desire for fame.

Tall and willowy, with a mop of brown hair and piercing blue eyes, Mr. Karp typically dresses in jeans, a T-shirt and sneakers. He speaks at a rapid clip and, often, for minutes without stopping. Technically, he never graduated from high school, which he cracked in an interview is “hopefully not a condition of Yahoo employment.”

After dropping out and working for a time in small New York tech outfits, Mr. Karp made his way to Tokyo, where he worked for several months for a start-up. He returned to the United States and became the chief technology officer for UrbanBaby, an Internet message board for parents. CNET Networks bought UrbanBaby in 2006, and Mr. Karp took the several hundred thousand dollars he made from the sale to start his own company, called Davidville. One of Davidville’s projects was a simple blogging service called Tumblr.

Mr. Karp’s run at Tumblr has not been without problems. He had trouble hiring in Tumblr’s early days, unsure how to even interview recruits. He often thought large companies were too big for their own good, proclaiming he could manage Tumblr with a team of four.