Well you can use social networks for driving traffic not for getting backlinks. You can get only one backlink with in profile or you can share your website links in relevant fan pages or groups.
You can use different social networking sites for back links by creating profile there, as well as keeping updating these profiles with quality and unique content on these social networks!
Social networks are very good source of traffic for a website. You can create a business page for your website, create a profile and post interesting content and link back to your site within your posts., join related groups and share your links with them.
Bummer, you used to be able to use FBML to get some link juice on Facebook but now those links are nofollow.
You share your link with others on social networking sites... this topic has been repeated easily 4 or 5 times since this new Social Networking forum has been opened, the search feature should give you some more results
I use big mikes socialbot by incansoft. I only paid $12 for it. Works great and only takes me a few minutes to post to over 50 sites.