I purposely linked to this YouTube video while teaching tonight. It's for www.nikefootball.com.
Why and what's the point? It was a bit of an experiment. We were trying to figure out whether the link would lead back to the original site. Some Social Marketing sites have figured out how to block SEO strategies for linking back to their mother.
Who cares? Well, if you can create a link back to your site, it will improve your natural (organic) search results - when someone types in keywords, you'll come up on top. But some sites, like Twitter, automatically insert code into your entries that tell search engines to ignore your link.
Again, so what? If you're working to improve your SEO rankings, then some social networks are better than others...Facebook, Linkedin, YouTube: Good. Twitter (and others to be researched): Bad. From a SEO perspective.
It's still a good idea to put your stuff out there to as many places as possible. Don't worry about SEO, exclusively. It's about getting your name out there and having people talk about you.