Great video and article from Alexia in TechCrunch in May 2011:

Says Karp on the new users, “90% come from the Join Tumblr button up in the corner of blogs, the Follow button on blogs, typing in Tumblr after you’ve visited a blog on Tumblr so you’re trying to investigate what this thing is that they’re using, or clicking the Powered by Tumblr link at the bottom of the screen.” This means that the other 10% are either typing in Tumblr.com directly or Googling.

Once registered 85% of users are still posting a week after sign up.

There are currently almost 20 million blogs on Tumblr, and that number seems to be rapidly rising. “Frankly, [there's] pretty little incentive to sign up,” says Karp. “It’s not like you have to register to comment on those blogs or to interact with them. But people see these things that people are doing with Tumblr and it inspires them to want to try that for themselves.”

Awesome engagement. Well played, Tumblr.

This technique doesn't work for every website.

But it really works well for Tumbledore.