One common reason is that it is a requirement to do the fast app switching Facebook auth, which is why you see many URL schemes in that list in the form of fb[f

One common reason is that it is a requirement to do the fast app switching Facebook auth, which is why you see many URL schemes in that list in the form of fb[facebook app id][optional suffix]:// This is so that after auth the Facebook app has a URL to fast app switch back to and handle the authentication result.