FB is one of the worst ad network I've had the displeasure to use.
First in terms of analytics and reporting. Adwords plays the "AWS billing card", they're transparent without being so, and the overabondance of unrelated metrics makes it sometime difficult to figure out where you are wasting cash. Still, it appears to be working hand in hand with me, they would like me to overpay what I need, but they also want it to make me money so I come back for more. They're my own personnal crack dealer. Facebook on the other hand really feel like a casino experience. It's basically telling you "it's working ! you should be pleased with the result !" and in general doesn't feel like a pleasant relationship to me. Doesn't surprise me in the least that they were caught many times faking numbers because that's the general vibe it gives off.
Second in terms of results. Talking to other people in the field it appears to perform decently for retargeting, or to grab a mass volume traffic/eyeballs. And they make lots of billions so clearly there is value to get there. But for high value qualified leads, FB is basically the same as Adwords "display network", which is already known as the crappy terrible part of adwords you should probably not use. Doesn't really surprise me, on Google/Amazon/Bing you're appearing in front of someone who typed "amazon I need a water pump" or "google I need a pdf editor", you're answering the user's question. Whereas on FB just like display network you just appear on content or for users that are vaguely related, you're bothering the user more than anything.
Like everything Ads it depends on your product and your market and how much time you can spend on it, but for me it's a massive dud.
Again, everything written here is my own experience and preference, I might be using it wrong, YMMV, lots of people make lots of money of it and all that.