I've never understood why the Facebook client is so heavy. That isn't a long list of features! It seems like the requirements could be written on the back of an envelope.

Obviously part of the reason is that they move fast and have a lot of developers, so there's much more pressure to add code than to keep it trimmed down. But the client still doesn't do all that much. Surely, surely most of the logic is handled on the server.

Instead of using the platform-provided, battery efficient GCM (now FCM), they insist on using their own MQTT push solution.

Not only does this require a constantly running background service and a TCP socket which has to be reopened on every connectivity change (e.g. a switch from wi-fi to mobile data): They do it for every single one of their apps.

My phone has at least three of them (Instagram, Facebook, Messsenger), and I'm pretty sure that this is a major reason for battery life problems on Android.