Ok, I solved this problem.
- add your app's bundle ID into the 'Native iOS App' tab under 'developers.facebook.com/apps'
- Your app's bundle ID should be the same with the one above (Facebook developer page)
- After I added up my bundle ID, I started to receive 'com.facebook.sdk error code=2' messages. It is good to check this link which explains everything step by step about the error code and how to get rid of it.
- So, in my specific situation following worked; Device's Settings => Facebook and then log in with the credentials.
I spent too much time on that but now it works!