1. Go to developers.facebook.com and create an app:

  2. Click Add Platform, choose Website, and enter http://forum.example.com in the Site URL field.

  3. Click on Settings on the left. Enter forum.example.com in the App Domains field.

  4. Enter your email in the Contact Email field

  5. Under the Advanced tab (at the top), make sure Client OAuth Login is enabled. Enter http://forum.example.com/auth/facebook/callback in the Valid OAuth redirect URIs field.

  6. Click the Save Changes button.

  7. The App ID and App Secret go in the facebook_app_id and facebook_app_secret settings in the users section.

  8. Go to Status & Review and change “available to the general public” to Yes.

Source: https://meta.discourse.org/t/configuring-facebook-login-for-discourse/13394