Dude don't waste your time resetting your phone. I am positive it is a problem with the newest Facebook update. I am running an iPhone 4 with AT&T, on jailbroken 5.1 and am having the same issue. I am going to try downgrading my Facebook app through Install0us and will let y'all know what results I turn up.
LOL, to late did a full restore and guess what? NOTHING! GRRRRR... However it is nice to hear that this is starting to happen to other folks on other carriers. Now i might be looking to far into things, but I think I am not getting notifications on certain apps now. Mostly game apps like draw something and song pop. Not sure if draw something ever made a sound, but I do know that song pop used to and it was the same sound that facebook uses. HMMMM!!!! Is it possible that the facebook apps notification noise has somehow been deleted?