Most social networks do not have any direct affect on your SEO purposes, as most of them use direct links (not keyword anchored) and/or URL shortening services. So there’s not much advantage to "getting a link" from these sites for that purpose.

You can, however, as someone suggested, submit content to Digg, Reddit, StumbleUpon, Delicious and others to generate traffic. Once again, however, you can’t really chalk that up to "SEO efforts", but it will help you increase traffic, which in turn can lead to brand recognition and perhaps a few sales/leads/clicks (depending on what your sites purpose is).

Lastly, I wouldn’t submit your content to these sites yourself, but rather focus on creating great content that others will want to submit for you. That way you’ll gain some possible long tail phrases as well as decent traffic, because people are genuinely interested in what you have to say.

If you are looking for forums I would suggest Digital Point