Though, there is such a question Facebook Error (7) iOS 6 it\'s already closed without any answer! While obtaining an access to user\'s facebook account I\'ve got an error:
A few more things that one can try other than the above suggestions is checking your app settings on facebook.com here -
https://developers.facebook.com/apps/<*APP_ID*>/summary
Make sure that the SandBox Mode is set to Off
Also,
Under "Select how your app integrates with Facebook"
Select-> Native iOS App
Save changes and try after some time as it might take a few minutes for facebook to sync the changes to all its servers.