As a copywriter I may be biased but I see unique content, in any form, as the real key to social marketing success. There are millions of potential visitors all being wooed by millions of quality sites that cater to their interests. Good copy and interesting content is a big draw, both through search engines and social platforms.
That being said, beyond Twitter (do any of you really find Facebook useful for marketing?), there doesn't seem to be anything comparable on the horizon.
From what I've seen, MySpace is pretty much dead. They're actually trying to turn it into some online game platform to revive it according to some article I read not too long ago, as for what's still alive and working, Twitter seems to be quite effective provided that your customer base is willing to give it a try.