We have Facebook, Twitter, Linkedin, Pinterest, Youtube and Google +. I need to say that the most traffic of course comes from Facebook, but it depends on your target market a lot as well. And it is important to know that it mostly helps for bringing traffic, but conversions are not as common thing.
We have Facebook and Instagram accounts.. We haven't seen a big difference yet, but we are investing in making some ads on Facebook for the forum.. The projectile says that we should have a few hundred visitors to the page daily.
Currently i am using Twitter, Facebook and google+, So far im finding most traffic is coming from twitter for me though as a post above the conversion rates are quite low.
What people don't realize in all this negativity in this thread, is that Social Media is now a ranking signal for your site, blog, or forum.
Revillution has a twitter, a reddit and a facebook page, even though facebook being hte nazi's they are, don't like the word "Revillution" so the page name reads Nucleus Boards but the URL itself is still revillution. It makes NO sense to me, but it's whatever.