Allow User to Login using Facebook
This options allows user to login to the site using his/ her Facebook credentials. This bypasses some of the settings of registration process on your site since data is pulled from Facebook directly.
This will also allow users to login directly from Login box page and register in the process. If a user has already registered on the site, and tries to login through Facebook next time, as long as his/ her email is same on Facebook (as on the site), he will be logged in through his registered profile. If the email on Facebook is different, it will create a new user account on the site using the Facebook email.
The whole process requires your site to interact with Facebook using Facebook’s standard protocol. To make it work you will need to register your site on Facebook and create an App ID and an App Secret. For detailed guidelines place see this link: