Is this on Android? Facebook deprecated login on Android embedded browsers, so that might be related to your problem. Here is a post from Facebook about this: https://developers.facebook.com/blog/post/2021/06/28/deprecating-support-fb-login-authentication-android-embedded-browsers/
Some things to try:
Use a different browser:
Try installing another browser such as Chrome, Brave, or Firefox. Then log in again with your Facebook credentials.
Add a new test user and see if you have the same problem.
I would recommend reaching out to Facebook support for this one as I don't think this is caused by Azure AD B2C. https://developers.facebook.com/community/