link to an article about publishing. This is a very informative and interesting article in Forbes Magazine by David Vinjamuri,

Publishing Is Broken, We're Drowning In Indie Books - And That's A Good Thing

Publishing is different from other creative industries because the machinery has not yet adapted to the profound technological shift it is undergoing.

...Most book sales are driven by personal recommendation. Social media – from Twitter and Facebooks to rating sites like Goodreads – just make it easier to recommend a book to lots of friends at the same time.

Hugh Howey says:

The effect of social media has been massive. Where social media is really powerful is in the area where the author has no control whatsoever. Word of mouth for WOOL spread because people were posting it on their Facebook page and Twitter accounts. It wasn’t through my efforts; but the efforts of readers. Many people think social media is about what the authors are doing not what readers are doing, and I think they miss the real driving force behind their sales.