Sincere request: Can somebody explain the dynamics of how Stumbleupon is so popular? I don't know anybody who uses it. I am friends with alpha-geeks, teenagers, senior citizens, native Spanish-speakers in Mexico, etc. Barely anybody I know uses it.

I had a StumbleUpon account before Twitter, before StumbleUpon was a URL shortener, etc. I used it a little, but people didn't seem thaaaat into it. There must be some pocket of main use-case there. Is it mostly the URL shortener that even non-S.U. users bounce around in when directed to a su.pr URL?

I'd love to understand this.

(Also, as somebody else pointed out, S.U. traffic is pretty low-quality. Just peep your Google Analytics, and those sessions are quite short compared to Twitter/FB/Tumblr referrers. At least on my sites.)