Facebook uses openid to eg, allow users to sign in with a google openid when signing up for facebook. But as far as I know, Facebook is not an openid endpoint,

Facebook uses openid to eg, allow users to sign in with a google openid when signing up for facebook. But as far as I know, Facebook is not an openid endpoint, you can't auth against a facebook account using openid. Happily, google is an openid endpoint, and basically everyone has a google account for gmail or something.