I have noticed digg only helped for traffic and getting indexed and never really saw much from stumbleupon. My question is how much does his help serps assuming I am using relevant anchor text?
I get lots of sales and traffic from Digg mostly due to organic searches. Most of the time, my sites will show up in Digg under the keyword higher then my site or right below my site. Either way, it gives you two or more chances to get noticed for the same article in the search engine. Digg has been great for me in this respect.
StumbleUpon doesn't affect SERP because it's NoFollow. IMHO social bookmarking has little to no SERP impact. Why should Google give value to crappy links?
I'm not saying it helps my SERPS, but when I check for certain keywords my site may be #5 but the post I submitted to Digg could be #3, which gives me two unique positions for one keyword. So in that respect, it helps me out a ton.