Oh, Facebook. The version released for iOS 9 is a real battery drain. Kind of technical reasons why this happened. If you are really interested you can read about it here.
If you have an iPhone or iPad, the bottom line is that you should update the Facebook app to the latest version. Many of you may have updates on automatic, so you’re covered. To double check, open the App Store app. Click Updates at the bottom of the screen and look to see if Facebook needs updating. If there are any other pending updates, do them as well.