Me I am using do follow Social networking sites.. Aside from Traffic it can be use also in SEO.. Check my list here List of do Follow Social Media Site
I think its all according to what you want from it. I use Digg and Stumbleupon plus lots more etc for SEO but don't really seem to get much traffic. I dont really spam them but submit and suggest lots of interesting stories. Guess they just dont like me! What type of traffic do you get? the type that you want?
Stumble upon tends to be untargeted traffic, however it does spread the word about your website so that the 1 in 100 people that comes to your website actually has an interest in the site. I suppose social media marketing works differently with every different market.
I'm getting site mostly from facebook and twitter as well. I don't know why I'm not getting traffic from Digg
SU, Digg and Delicious are great for english sites, but unfortunately they bring almost no traffic to my ukrainian site.
Mainly from Facebook for me. My website has a big fanpage and quite a few of the users use facebook - plus you gotta use it or twitter to register so naturally all users must have one