I'm going to go with facebook. (not for backlinks though... and don't forget Myspace ads no follow on links too).
I like Twitter for getting lots of traffic easily. Facebook is good, but you don't want to spam people there.
For social networking Facebook is best of all, although Google+ is trying to compete it but far away.
Social networking sites are primarily not for backlinks. The best one is facebook and google+ seems to be quite promising. We will see in the future what happens. ANyways if you are looking for backlinks from social sites you should be trying out those link sharing portals like digg.
If backlink is only concern than Digg would be the best. But if you need traffic then Facebook and twitter are the best
Beside Facebook and twitter, pinterest is chasing up to become one of the popular social networking website