Adding to David’s post: “Mac asked how the ‘Top Tumblelogs’ on the Tumblr home page are chosen. The algorithm behind it is actually really boring, and only based on the raw traffic numbers.”
It’s not quite that boring. We run a number of filters to detect “good” tumblelogs, and filter out the ones that are obviously spammy. And every day, we go through them and remove any that shouldn’t have passed the filters or haven’t been updated in a long time. And we combine RSS readership with tumblelog readership in an intelligent way.
But David’s right: this will be dramatically updated soon.