Is it ok to have adsense on sites that get you tons of hits from stumbleupon? idk if adsense likes that or if they can recognize where the traffic is coming from.
I would recommend using an alternative ad display on pages that get StumbleUpon traffic. Yes, Google knows where your referral traffic is from.
I don't think Google should have any kind of problem with StumbleUpon traffic, they just dont like PopUp and Pop under Traffic thats it Even I get around 1000-2000 Hits/day from Stumbleupon for about 1 week now and earned around $25+
You won't monetize much from it. Plus, if your page is full of AdSense, people will stumble you down and the visibility you hope for is close to being impossible. I actually reported on this last week: http://www.seroundtable.com/archives/013173.html
Since StumbleUpon traffic is not autosurf or traffic exchange related traffic so it should be ok. Same for del.icio.us.