I've touched on this before. I see lots of authors effectively buying likes through Facebook and paying for sponsored posts. I shake my head at this. Here are the reasons you should NOT be paying for Facebook advertising.
~ Every penny you spend on marketing should be an investment. You are investing in an indefinite thing. For all we know, Facebook could close tomorrow. Their priority is their stockholders, not you or the user. Why would anyone invest in something with no long term benefit?
~It's not good value. You are paying to reach people who may have shown some kind of interest in books. Like myself. But I'm probably not going to buy your book because I'm too darned busy. If you're going to spend money, invest in places where you know book lovers hang out and with strong statistics. Most book blogs and email lists can tell you exactly what their reach is. Facebook only give you an estimate.
~Sponsored posts annoy people. By sponsoring a post, it may actually have a negative effect. Many people actually mark them as spam because they come up in their newsfeed. Do you want to be seen as an annoying author?
*Note, I do still advocate having a Facebook page at the moment. It works for many authors, particularly contemporary ones, but it is absolutely not worth paying for.