Unfortunately, after 5 years of using Login with Facebook and never having a single issue, Facebook decided to completely block all of our domains without any notice at all. We have been banned for allegedly violating their community standards, which have certainly shifted much over the past 5 years due to their various blunders.
Going forward we will no longer be offering any 3rd-party logins, and will reduce our dependence on other 3rd party services whenever possible to avoid these inconveniences.
We're very sorry for any difficulty logging in. Just know that it is completely Facebook's fault, they gave us no notice and just turned our sites off, and there is no way to get a hold of Facebook to appeal their mistake.