So friends, I am pretty sure that some of you are totally annoyed with your inability to deactivate your facebook account because you have your favorite webapp associated with Facebook for login. This sucks to be honest.
Ok, chances are that the DB of the webapp that you want to use has your email ID set as a regular user and Facebook association is only a FK relationship. Good news.
In simple words, you can actually (99% chances) detach your favorite webapp from a social site by just resetting the password using the Forgot Password option. I tried it with Duolingo and it works haha.
So, go ahead n deactivate facebook peacefully and still log0in to your favorite webapp. Good luck!