With Facebook Login option, you can provide your customers with a secure, fast and convenient way to log into your website without them having to create a password. To get started, visit https://developers.facebook.com/
Create or enter your account on Facebook for developers and click Add a new app:
In the Products section add a new product and select Facebook Login. Click Set Up.
Go to Settings in the left sidebar and add your website URL: yoursitename.com/userlogin/?social-login=facebook in the Valid OAuth redirect URIs field and click Save changes (you may leave all other fields blank).
Go to your Settings => Basic. Copy your App ID and App Secret.
Paste them in your WP admin area: AliDropship => Settings => Accounts.
The last step is activation of your App on Facebook. Go to App Review and make your website public. Click Confirm to finish.