Make sure that your URLSchemeSuffix in your Info.plist is correct (fb[appid]) and that the handleOpenURL call (in your generated XCode project) is making it back to facebook.
I had the very same problem where another plugin was capturing the handleOpenURL call when i was returning from facebook after authorizing the app and it was preventing the handleOpenURL response from making it all the way back to the facebook plugin.
You can test it by putting a breakpoint in the Facebook/FbUnityInterface.mm file @
- (BOOL)openURL:(NSURL*)url sourceApplication:(NSString*)sourceApplication
If this isn't being hit, it's likely another openURL handler is getting the response first and not handing it back to super.