Social media marketing is used more for specifically gathering traffic. It wont give dramatic results in terms of SERP and
PR boost effectiveness compared what it does to traffic. It's a great addition to any traditional SEO work that you do, but it’s not a substitute. On-page SEO is definitely not as sexy as social media marketing, but it is still the most important investment in your website that you can make.
here is an article relating to SMM and SEO.
http://searchengineland.com/social-m...-new-seo-12531