See, e.g., Arvind's comments on StumbleUpon visitors:
http://arvindn.livejournal.com/133249.html
To quote the takeaway, "most of the traffic generated by StumbleUpon users to any given site is going to be low quality because the dopamine junkies make 100x more clicks".
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.)
1 Facebook user might click on 1 or 2 links per day.
1 SU user "stumbles" dozens or hundreds of times in a session.
A stumble nets a pageview, but obviously has considerably less value (no intent baked into the click).
It's like a youtube party, but with a little fast-forward button: http://xkcd.com/920/
Worth noting that SU traffic may be unusually low-engagement, since I would think it's likely a user will just click Stumble again after the half a second or so you get to grab their attention. (That's an educated guess, mind you--I don't have any data to support this beyond how using SU felt to me.)
And yet I know so little about it. I know they have something like 10M active users, which seems really high.
Compare: http://i.imgur.com/oY03q.png
It's been like this since 2008.