Attempting to log in to secure, at least, with a Facebook account is currently failing:

Unhandled Exception ("HTTPFutureHTTPResponseStatus")	
[HTTP/400] 
{"error":{"message":"(#100) Tried accessing nonexisting field (security_settings) on node type (User)","type":"OAuthException","code":100...

This (probably?) worked as of October 20 2020:

https://discourse.phabricator-community.org/t/fb-login-broken-on-secure-phabricator-com/4320

The security_settings field may have been deprecated/removed on November 20 2020:

https://developers.facebook.com/docs/graph-api/changelog/version8.0/

A likely fix is to stop using this field, which has probably become "always on" (I believe it was a "make things actually secure" checkbox, that was a checkbox only for backward compatibility reasons).

Another fix is to delete Facebook auth, since it seems like no one is using it in the wild if it has really been broken for several months without a report.