Social media ties in well with SEO marketing. They are mutually beneficial.
I have a new site where social media links helped push the site up in the search engines. Only links are from my other sites. And Twitter and Facebook. In a week it's in the top 10 of its main keyword. Even went up to #1, but holding steady at number 4. I don't think I'd rank well that fast without using social media first to jumpstart the site as it is the easiest way to get links and site noticed by other people.
The bonus with SMM is that you get the feedback immediately about your content. I've had one of the acknowledged thinkers in our industry praising a post I did on Twitter, and as a result, that post got a lot of RTs, trackbacks and links from other blogs and sites. That showed me there's something worthwhile in SMM