The pair coded a crude version of Tumblr — it was in private Beta in October 2006, and was released as a free tumblelogging platform in early 2007. Within the first two weeks, 75,000 users created blogs. “The one really cool thing was we could let you change anything about its theme,” says Karp. “It was just a big chunk of code you could rip apart and make original. And it attracted this spectacular community of designers and hackers. Over the next few weeks, they built gorgeous things on Tumblr that didn’t look like anything else on the internet. That still defines Tumblr to this day.
— Tumbling on success: How Tumblr’s David Karp built a £500 million empire (Wired UK)
