I suspect Tumblr beat out Posterous because of its community features. By showing a feed of new posts, a la Twitter, it aided the discoverability of content. An

I suspect Tumblr beat out Posterous because of its community features. By showing a feed of new posts, a la Twitter, it aided the discoverability of content. And there's nothing to spur on a community like the addiction of getting likes, notes, and reblogs from other people.