It's funny enough that facebook can't possibly access your "other" messages without forcing you to install facebook messenger (which end up being viewable in the facebook app itself), but you can't turn off notificatinos permanently on facebook messenger. Any time you click the box which says "turn off notifications" instead of just doing it , it says "okay, we'll turn off notifications for 8 hours and then turn them back on automatically for you"
wtf??
Talk about dirty pool.
Luckily newer versions of android allow you override this and just stop receiving all notifications from an app permanently.