An illuminating post from Michael Alvear at Digital Book World:
As an author, book marketer and social media specialist, I cannot think of a single more wasteful thing an author can do for book sales than to market on Facebook. Put simply, there is no evidence that Facebook can sell books, unless you’re a celebrity with a mass following. There is, however, plenty of evidence that Facebook is both a waste of time and money if you’re an unknown or midlist author.
To understand why Facebook is so demonstrably bad at selling books, you have to understand two key concepts that agents, publishers and marketing experts fail to mention whenever they encourage (and sometimes force) authors to build their “platforms:”