If you have the Facebook App on your iOS device, and you download the Facebook Camera App, the first time you go into Facebook Camera, you get:
Continue as David W.
(or whatever you're name is)
In iOS, all apps are suppose to store documents under App specific folders. There is a keychain for storing passwords and other credentials, but again, this is application specific. I even found documentation explaining that although Safari can download certificates, they must be imported into particular apps. The idea is to sandbox each app, so a security issue in one app doesn't affect the security of the entire device.
So, how does the Facebook Camera app know who I am if it can't get data from the Facebook App itself, and can't use Facebook App's data from the keychain?