Technically, SEO and social media marketing are two different things. However most people looking for SEO really just want an improved web presence that results in traffic, leads and sales, regardless of the source.
SEO, strictly defined, focuses more on keyword optimization and website architecture. However, a good SEO (aka web marketing) strategy must be coupled with other web marketing services in order to succeed. One of the most important of those services is social media marketing.
Link building used to be a significant portion of SEO. Today, however, social media has pretty much replaced that as the go-to method of building relationships that turn into not just links, but added promotion and exposure for your company. The signals that are produced through a strong social media campaign are valuable to search engine algorithms and, ultimately, lead to improved site performance in those engines.
While SMM doesn’t produce a pure one-on-one correlation in the algorithms, the overall impact can be pretty substantial. So much so that I would be hesitant to trust any web marketing campaign that doesn’t allow any room for social media marketing.