Facebook used to be a great place to advertise books, simply by posting about them, if you had a professional page. But Mark Z realized he could monetize this, so now those posts only go to a small group of your Likers. (You don't get Friended on a professional page; you get Liked.) Then $5 would get you a thousand pairs of eyeballs, which was probably worthwhile. Now it's $20, and it's a waste of money.