For starters it is not "Selling" it is a way to engage your visitors, help promote your brand. But most of all it is about being SOCIAL!!!
Social media itself is a catch-all term for sites that may provide radically different social actions. For instance, Twitter is a social site designed to let people share short messages or “updates” with others. Facebook, in contrast is a full-blown social networking site that allows for sharing updates, photos, joining events and a variety of other activities.