That’s an extremely hard thing to do. Even if you manage to do it, the sales do not come immediately. The reason why it is hard to turn Facebook fans into book buyers is because 1. They are your friends and family who, let’s face it, tell you they’ll buy your book but never do. 2. They already own your book and that is why they are a fan.
So instead, you have to think of your Facebook fans as your ready made reviewers and purchasers of your next book. Your Facebook page is full of people that already like you. I would recommend offering them discounts or free copies of your next book in exchange for reviews. Do this early, at least two months before your next release so that you will immediately have some reviews when you release. Having reviews will give you the greatest chance for increasing sales.