With the release of iOS 6, facebook is integrated into the iOS environment. In the previous version, if you needed a user to login into fb, there was an app switch which used to take place.
But with 6, if the user if logged into fb via settings, the OS authenticates it directly without any app switch.
For this to work, you would need to specify the bundle id in the facebook app settings.
If you have two or more bundle ids, you can specify it as below, so that you use a single fb app id, for mulitple apple apps.