To display the button you must absolutely enable it using the option Enable Facebook Login.
Now, you are asked to enter “Facebook App ID” and “Facebook Secret” to allow your users to log in via Facebook.
To recover these information, you have to create a new application in the page https://developers.facebook.com/apps.
Create a new application by clicking on Add New App, that you can find in submenu My Apps.
Then, you’ll see a screen like the following. Set Display Name and Contact Email to associate to app.
Select “Set Up” for “Facebook Login”.
Select “Web option.
Set your “Site Url”
Skip all others step of the process.
Open “Settings” and set in the option “Valid OAuth redirect URIs” the url that you will find in the “Facebook” tab of the plugin settings, than save the settings.
If you have followed the procedure step by step and you have inserted “App ID” and “App Secret” in the plugin settings, you have completed the configuration for the login through Facebook.
You are now ready to use it.
This message shows only when logging in with the same email address used to create the Facebook app.
On WPEngine hosted sites you should also ask WPEngine to exclude from caching the path wp-content/plugins/yith-woocommerce-social-login-premium/includes/hybridauth/ and the cookie ywsl_wp_session and add the following line in the wp-config.php file:
define( ‘YWSL_FINAL_SLASH’, true );