You said add comment when you want to ask related questions. But I see that my questions are not responsed when I add comment! So I decided to open a new thread.
I guess I understood different thing from LinkUserFacebookAccount. I want to use facebook a bit different. I want user login into my game with userService(username,email and password). Then I want to offer user to link his facebook account to his normal(userService) account. So, next time when user wants to login, he or she can loggin either using his username and password or with facebook. When player choose to login with facebook, I will again get user informations as if he logged in using user service. User did not link his facebook account to any user service account but still try to log in with facebook, I will prompt "Your facebook account is not linked to any account!". Do I think different then how it is used? Or how can I achieve what I say?