The iOS developer behind Home Remind has published a blog post about the Facebook apps for iPhone, iPad and Facebook Messenger. He says that according to his testing, the Facebook apps consume way more CPU time than is strictly necessary. Excessive CPU time can lead to battery drain.
The developer used Apple’s own Mac-based app, Instruments, to look at what was running on his iPhone, and found that his Facebook app was activating, doing something for ten seconds, then going back to sleep. It did this all day long during his test. He tested the Messenger app and the Facebook iPad app, and found the same pattern.
If that’s the case, the Facebook app is never truly going to sleep and then terminating like a good app. As a result, it’s using up CPU time, and a lot of your battery.