Hello,
1) Check your Facebook settings according to this video – https://xstore.helpscoutdocs.com/article/87-facebook-login and docs https://developers.facebook.com/docs/development/register
2) Try to disable all the third-party plugins (including cache plugins), that do not come bundled with XStore https://xstore.helpscoutdocs.com/article/36-included-plugins , clear cache of browser, and check how it works then.
Regards