The chart below, made by Statista (link is external) from Shareaholic (link is external) research, shows Facebook is king of the traffic referrals among Shareaholic's user base.

Pinterest comes in second, way out in front of much older Twitter.
We think Pinterest is just getting started as a commercial traffic driver, so this doesn't surprise us.

The Shareaholic article (link is external) covers the rest of the pack too- StumbleUpon and Reddit decline in referrals, YouTube and LinkedIn gain share at a decent clip.
Google+ is stomped by the competition, basically flatlined on the graph. However, there's a lively discussion in the article comments, about why marketers still can't afford to ignore Google+. We found this article (link is external) by Mike Allton important reading, especially the quote below.

Articles that conclusively prove some social stream is #1 are everywhere (link is external). And new contenders are constantly being held up as the next big thing (see this article (link is external) on billion-user app WeChat!)
But we still believe what Dorie Clark said (link is external) in Forbes last year: "No One in Marketing Knows What They're Doing". We say make good, Hummingbird-friendly content, even though you never wanted to be a publisher, and stay alert.