Facebook Login is a Magento 2 extension allowing your customers to login or create an account at your store using the Facebook account.

Run composer require scandiweb/facebook-login

For Facebook Login to work you need to Setup Application on Facebook and obtain App ID and App Secret.

1. Go to https://developers.facebook.com/apps and create a new application by clicking Add a New App
2. Fill out any required fields
3. Go to Settings tab
4. Click on Add Platform and choose Website
5. Put your website domain in Site Url field

For more details please read Register and Configure an App

Once you obtain two keys navigate to Magento Administration, copy/paste them to appropriate fields under Stores->Configuration->Scandiweb->Social Login->Facebook Login, set Enable Facebook Login to Yes and you’re good to go.

You can see the screenshots following this link