I reckon Twitter is personally quite good. I've used it mostly for blogs, but I do find if you use the right hashtags etc that you can get some people making their way across to your site.
Twitter, Reddit(if you consider it a social site), Facebook and Tumblr. Now the hard part is to promote your content in a way that will give you the best first impression. Social Networks promotion can be hard if you are promoting a forum.
Probably twitter, but dunno. I think forums are just as good as social networks, particularly for promoting forums. You don't have to sell people on the idea of using a forum in general, and I think the engagement between members is much more authentic on a forum than on a social network.
I'm not really sure, but I use Twitter and Facebook primarily. I have not tried Google's targeted ad campaigns though. I plan to and will let you know how it goes.
I still have yet to really push advertising on social media for my forums however I've started to use Facebook for getting the word out there about my blog and that has been doing wonders for my traffic stats.
I think Facebook. Links and text used in status messages can reach out to a lot of people if you have many friends. Making a group for your forum/site may be very effective too, as you can add major updates and changes easily.
When I still had a forum, Facebook helped me most. We created a group at Facebook and got members interested in the group. From there they checked the forum and became active at the forum too. But we created the Facebook group after we had some members at the forum, though, so there could still be activity in the group. Perhaps it wouldn't work as good if the group started empty.
According to me Facebook is one of the best social media site. Facebook is the platform where you can easily share information with your friends. It also helps to increase backlinks and more traffic to your website.