Hi,

As partially answered above, if you go to your User Centre you can bind your account to an email address there. Keep in mind this email cannot already be attached to an existing Naruto Online account. Once you have bound your acc then you will be able to login via the email address rather then facebook and if you were to delete your facebook then your account would be unaffected. You will however, to answer your question still be able to login via Facebook.

Best of Luck