Yes, it can sell books. But the sales benefits are far down on the list of benefits that you will accrue using social media the right way. More likely: You are making friends, you are learning about what else is out there, you are exchanging knowledge, you are discovering, you are communicating, and opportunities will come your way as a result. So, yes. Social media does not a bestseller make. It's never going to match the effectiveness of a national media campaign. It's never going to match the efforts of a dedicated publisher's marketing efforts. - danblank