To activate the Facebook Social Login module you have to set the Facebook App ID and Facebook App Secret Key under 'Administrator - Settings - Users - Facebook Login Settings'.
The required App ID and App Key can be obtained at https://developers.facebook.com/apps
Click 'My Apps' in the top menu and the 'Create App' button that will appear. This will open a popup. Choose 'Build Connected Experiences' and click 'Continue'. Set the application display name, contact email and business manager account (optional) to be associated with the app. Click 'Create App' when ready.
Choose 'My Apps' in the top menu and click the social login app icon to enter its settings page. In the very bottom of the dashboard page find the 'Facebook Login' in the 'My Products' section. Click 'Settings' for this 'Facebook Login' card.
Under 'Client OAuth Settings' enter your redirect URL in the 'Valid OAuth Redirect URIs' field. The URL must include your domain name and path to the phpListings authorization controller, e.g.: https://demo.phplistings.com/account/callback/facebook Click 'Save Changes'.
The 'App ID' and 'App Secret' can be obtained from the app page now. Use these in your phpListings settings page as described above.