There are certainly some products where face to face interaction is critical in the selling process. However, with more people doing research beforehand on social media sites and seeking out opinions/reviews on products, will marketing have to monitor those areas to make sure the correct message and information can be found. This role of gatekeeper seems more suited for marketing than sales to me.
Marketing's key role is communicating with the customer until they are ready to buy. Social media sites provide a great venue for this conversation. I do not think it is too far fetched to think that marketing could become a key player in turning prospects in customers without interaction from sales.