Deeplink to Facebook App (using fb: protocol) not working from Facebook in-app browser
Apple apps are sandboxed. This means they cannot access other apps and execute code. Facebook is running a browsing instance and when you try to call the fb:// protocol, the iPhone is blocking you from doing this to try to create an infinite app loading loop. I.e, you open a page in FB browser and it opens itself in FB browser and it opens itself in FB browser...