I don't know the problem is part of the template not of the facebook login.
The error is the following included in the template, probably they are attempting to do a wrong operation:

PHP Fatal error: Call to a member function init() on null in /home/okhalalb/domains/okhalal.biz/public_html/templates/easyshop/error.php on line 15

Also i tried your website through a FB login and it works with no errors at my end.
You have to contact template developers to fix definitely however.
John