A SEO firm is a technical firm that tries to make your website crawler friendly. If this was the case, one should ask why some companies hire SEO firms to organize their social campaigns.
According to the writer of the following article, the fact that most SEO companies do not offer link building services only mean that link building is more of a marketing issue than a SEO one.
True enough, a SEO company is a SEO company--and not a PR firm.
Link: Is There a Link Between SEO and Social? - ClickZ