I've read about security best practices saying that inactive user accounts should be disabled and even deleted to avoid security issues like unauthorized use. I can see that being true for regular username and password authentication sites, however my application was built to work only with Facebook groups and as such the only way to login or create a new account is to use the Facebook login.
The argument can be said that someone malicious could take control of one of my users' Facebook accounts and then use it access my application. Although that is true if they have control of a Facebook account my application would never know it's a malicious person so I don't see that as a valid criteria to use in determining if the account should be disabled.
Furthermore if a user is inactive and wants to become active again since it's Facebook login there really is no reason for them to go through some kind of reactivation process like confirming their email or changing their password.
I must be missing something here because it's certainly mentioned as a best practice to disable accounts but since my only login method is Facebook (OAuth) I can't come up with a valid reason to disable/delete inactive accounts.
Regarding other methods of unauthorized access I have security measures in place so I'd like to keep the answers relevant to the login method.
Please enlighten me if I've missed something.