First for Facebook Login you need to create the Facebook APP. Follow these steps.
At the beginning go to https://developers.facebook.com/
Login with Facebook > Add a New App.
Select a platform the website to get started.
Go through all the stages of setting
You can get APP ID and APP Secret from Dashboard.
Go to Theme Options > Social connect > Facebook and fill in the APP details:
Finally you need to edit Sing in page > Shortcode by Visual Composer > Sing in form Setting > Show facebook connect button > Yes