I'm a little surprised to see StumbleUpon so high! I remember using it when it came out, and I never really got hooked. Now that I think about it, it makes a little bit of sense. I hypothesize that the target market is different than sites like Hacker News, Reddit, and even Digg- StumbleUpon requires less effort and less contribution to a "geeky" community. I'm not saying this is necessarily a good thing, but they have found a way to get user submissions from people who would never get themselves involved in other social sites, so it's working for them.
Anyone have thoughts on this theory?