Facebook Connect required data is a webpart which allows you to join Facebook user ID with existing user account. If the page where this webpart sits is set in the site settings (Required user data page), user who signs in by Facebook Connect logon webpart is redirected to this page. If user already has an account he can use existing user part and login with his current credentials. User's Facebook ID will be joined with the account. Otherwise user can use the other part to create new user.