For most companies, I think their idea of social media marketing is setting up a FaceBook page. Does it work? For most, I'd guess not. But it doesn't cost much to try.
What I would really like to see is some actual case studies showing how social media worked for company XYZ, quantifying what it actually did (sales or profit), and why it worked. Until then, I would be skeptical, and wouldn't invest very much in it.