I just discovered this site: http://findasocialnetwork.com/search.php Type in the niche that you are operating in and it will give you a whole load of social networking sites relevant to you! I just discovered it and so I'm not sure just how great it is but I'm guessing you can create some really great relevant backlinks from there.
Well I did a search for "poker" and it came up with a bunch of social networking sites. They aren't the next facebook or anything but they ARE relevant and tend to have DOFOLLOW on all their pages. Most are likely to be PR0 or PR1 at best, but the sites are HIGHLY RELEVANT and will actually get you some good referral traffic.
Most of the sites I found provide NoFollow links only. It depends on your niche obviously. For some it should be an easy way to get a few related quality links. Cheers!
i used http://socialmarker.com/ few times where you can submit to many Sb sites at once. you however need to create account for all the sites for the first time. using firefox helps as it remember some id/pw but still there are some manual work involved.
I use socialmarker as well. It is an amazing site however I am referring more to social networking in this thread. There are tonnes of people in rip off sites of facebook that are just waiting to be marketed too. Some of these rip off sites actually have a load of members and DoFollow links and relevant traffic.