Though I can't say I use it, because it seems like a huge time suck, here's a good article on Stumbleupon traffic:
http://blogs.forbes.com/oliverchiang/2011/01/03/stumbleupon-passes-facebook-as-top-source-of-social-media-traffic/
Web discovery service StumbleUpon has just overtaken Facebook as the No. 1 source for social media traffic in the U.S., according to analytics site Statcounter. StumbleUpon accounted for about 43% of the U.S. social media traffic on January 1st, while Facebook accounted for about 38%.
Statcounter tracks 15 billion page views on the Web per month on more than three million websites. Among social media sites, it tracks the top seven: StumbleUpon, Facebook, Twitter, Reddit, YouTube, Myspace and Digg. Just last year during the same period, StumbleUpon made up 30% of the traffic, while Facebook had 48%. But, as a recent Atlantic article reported, the growth in online sharing with StumbleUpon outpaced Facebook in 2010.
So, if anyone uses the stumbleupon toolbar/bookmarlet be sure to give permies a thumbs up while you're on here.
It's not an installed toolbar like those spammy ones from Yahoo & Ask.com. It shows up inside the window. I think you might need an account, though.