Facebook login was one of our least-used features by customers, and required regular maintenance to keep it up to date with ongoing changes at Facebook. We ultimately decided to stop supporting it so we could focus our efforts on more important features.
If you logged in with a Facebook account in the past, you can use the Forgot your password? link on the sign in page to have a new password sent to you. Or, if your email address is a Gmail/Google account or Apple account, you can also use those login services instead.