I gave up with the FB app when it suddenly started taking 200MB of cache and upwards....
Think it was when the offline view came in, suddenly my phone ran out of internal storage all the time. As I have no intention of upgrading the (Android) phone I just deleted the app and now use FB through Chrome browser or Opera Mini, which give the same functionality as the desktop version including messages, so saving nearly 300MB of internal storage. Notifications still work, too.