you can try this code
in to the qa-page-register.php
if (QA_FINAL_EXTERNAL_USERS)
qa_fatal_error('User registration is handled by external code');
add the below code in place of upper code
if (QA_FINAL_EXTERNAL_USERS)
qa_register_plugin_module('login', 'qa-facebook-login.php', 'qa_facebook_login', 'Facebook Login');
not tested yet but i hope its work
Note: first activate your facebook plugin