aristotle - 6:35 pm on Jul 21, 2012 (gmt 0)
From what I know, Stumbleupon shows your site's page in a frame on their site, so that the "visitor" doesn't actually come to your site. Also, I once did an analysis of the "visitors" from stumbleupon to one of my sites, and found that most of them just take a brief look and then go to the next stumble, so there's much less engagement than with real visitors. I suspect this would be even worse with paid discovery. So it's hard to know exactly how Stumbleupon "visitors" would affect your Google rankings, but in my opinion the result would be more likely to be negative than positive.