I am having similar problems with my 6S+, but I don't think Facebook is the issue because the battery drain still occurs when I totally close the Facebook app. Interestingly the battery drains abnormally quickly when the device is in use, but not when the device is sleeping.
When my new iPhone 8 arrives I am going to run the 6S dead flat, then factory reset it. If that doesn't resolve the issue then I will call Applecare as it still has three weeks of coverage remaining.
I will post here if I make any useful discoveries.