You're going to need to open the files of your flarum installation, and edit the config.php file so that 'debug' => true instead of 'debug' => false. That should put your site in debug mode and show more descriptive error messages. Don't forget to change it back when done.
Also, go to your Flarum admin panel, then to the extensions page, and check the settings for the facebook auth extension to make sure they're correct.
