Social Media and Search Engine Optimisation maybe 2 different things as many are saying, however they complement each other.
The idea of SEO is to have higher ranking on the search engines. That is great but one of the important things that search engines love is the credibility of the site they are indexing. The credibility has been measured in various ways over the years some metrics were traffic and back-links (deep linking, one way linking, reciprocal linking, etc).
Today the same metrics in a way apply, but with much stricter rules. It is no longer the quantity of the links nor the volume of the traffic on its own. Today metrics such as where the traffic is coming from, the quality of the content where a link was found, etc
So if you think Social Media then think best traffic and quality of links, of course if your reputation is good.
This is really simplified I guess and how it is really measured by the search engines is really complex.
So think of SEM activities that provide interesting, engaging and quality content to your followers and hence the result is traffic, bookmarks, sharing with friends and better SEO.