Android: A recent server-side update to the Facebook and Messenger apps caused a bug that rapidly drained users' batteries even more than those apps usually do. Fortunately, it's been fixed, but you'll need to restart your apps.
According to a product manager at Facebook, the server-side update occurred on January 10 and has been affecting users for a couple of days. The update caused many devices to constantly use their CPU at high frequencies, using up battery power and generating a lot of heat. This even made it harder to use fast charging.
The server-side bug has been fixed, but you might still need to restart those apps. You can do so by heading to the Apps section of your Settings, finding the two Facebook apps and hitting Force Stop. Or you can wait for your phone to die. Better yet, you can just use the Facebook mobile site which can't drain your battery when you're not using it.