I get most of my social networking traffic from Google + and lesser from Facebook. Not sure if the traffic coming from Twitter to my forum is real people or just bots.
Right now - I am getting about 30% of my traffic from Google + and Pinterest. I never thought Pinterest would be that helpful.
I get over 80% of my traffic from Twitter... but then again my website is a Twitter toolset so that's obviously not typical.
I get 20% traffic through social bookmarking. I use Facebook and Twitter sites for bookmarking. I am increasing this traffic very soon.
Actually I once own a Facebook Fan Page, since people had more interest then I created a website. So my site threw visitors 50-70% from there.
Actually most of the people comes from Facebook but also many visitors comes from stumble upon as well.