If your Facebook battery usage has been especially bad recently, it has apparently been fixed server-side.
You don't use Facebook and also get surprised by battery drain issues, especially with the ocean of data to support just how hungry both Facebook and Messenger can be, but recently several users noticed a dramatic spike in power consumption. Some users reported as much a 1% every minute when Facebook Messenger was open, and it turns out that's not the intended battery consumption rate for that app.
Facebook's Messenger lead David Marcus responded to a recent complaint about power consumption on Twitter recently with an explanation and an apology, and it looks like whatever was going wrong has been fixed.
Facebook itself is still a significant battery hog, but even when you're playing games with friends Messenger is usually not too bad. As the tweet suggests, if you're still experiencing higher than usual drain from Messenger, force close the app and restart it so the new connection is no longer affected by the server-side issue that has been addressed recently.
If you feel compelled to do this more than once because of continued battery drain, chances are you're experiencing something entirely different from this problem. Fortunately for you, it looks like the guy in charge is pretty responsive.