I've been doing heavy research on the issue. Any possible resolution I've found (for example, when connected to WiFi, add 8.8.8.8 and 8.8.4.4 to the list of DNS servers) does nothing. Deactivating Facebook from a computer and then logging back in to the app to re-activate; deleting the Facebook app from the list of apps that are allowed to access your account from facebook.com, deleting the app, re-downloading the app to the iPhone and logging back in... NONE of these work. (Any of them work for you?)
I'm at a loss, and Facebook seems to be scratching their head. This is not an Apple issue, as it is a 3rd party app, and this issue started completely randomly on it's own back at 5.1.1 with nothing being added to the iPhone/changed....