What many people dont realize when the have a login access that if you are sharing the same Facebook app ID across the multiple iPhone apps things do get confusing. An example of such a case would be, having a paid and a free app sharing the same Facebook app.
Facebook would not know which app it should redirect too.
Solution is quite simple.
1. Distinguish your app:
Add a suffix ('paid' or 'free', depending on the app) to your URL Schemes setting for Facebook. You can find this configuration in your Project->Target->Info->URL Types property
- Distinguish your app in Facebook App dashboard:
Just add the 'paid' and 'free' in the app setting for URL Scheme Suffix.
Remember, the order at which you add your suffix string is important, since Facebook will try to find in the order you have entered the suffix string.