Facebook and Pinterest. I don't know if Facebook gives backlink authority but the amount of visitors you can get a network that huge is madness.
Facebook and LinkedIn are the best social networking websites in terms of for getting Backlinks and for generating the traffic to the website.
I like Google+ since the community just seems to be more mature. I also have the largest following on Google+.
1. google plus 2.facebook 3. meet me aka myyearbook 4.twitter 5. myspace only other one i can think of