Google plus is the best social network for backlink I think. So you will get a high pr backlink from google plus
Google+ is the best social network; not only for this particular reason but for all sorts of reasons.
Get quality back-links for social networking site Pinterest, ask.fm, digg, reddit, diigo like that...
Every social networking site can get you backlinks, the problem consists in the fact that the backlinks may not of great quality. Of course social media sites can strengthen your brand name.
I like results with Imfaceplate.com, for backlinking. Strong links, indexed very fast. Not sure about do-follow, but these links improve your rankings for sure. Twitter is good too, if you can squeeze your exact match keyword in the account name.
Facebook is the best social networking website. To increase network one should make friends and friends friend so on.
According to me, Twitter, Google+ and Stumbleupon are good to get backlinks as well as traffic to your website.