Yeah, currently use Twitter, Facenook and Myspace and seems to pull in plenty of Traffic. More so with Twitter!
I don't.. I think that it would only bring the wrong kinds of people to my forum, and so I avoid this at all costs.
I personally don't have a social networking account, but I asked help from my members to post it on their facebook and twitter account.
Social networking is a big source of getting high volume of traffic and backlinks as well. If one use social networking in a good manner then sire will be promoted in search engine as well.