There is an issue with Facebook authentication and Joomla 3.8.4. We believe we've localized and fixed the issue and will have a release out over the next day or so to fix. If you'd like us to implement the changes on your site, please Private Message us FTP credentials and we'll get you going. We're performing further testing on our end to validate the changes, but more tests on actual user sites is welcome to ensure the fix does what we expect it to do.
You can PM me by clicking the mail icon under my name to the left.
Thanks,
Alex