With the WP Discourse plugin you can configure WordPress to be the SSO provider for Discourse. This means that all authentication for your Discourse site will happen on WordPress. An example of how it works is that if a user clicks the Login button on your Discourse site, they will be automatically redirected to WordPress, taken through the login process there, and then be redirected to Discourse.
If you enable SSO, social logins will only be in place on your WordPress site. If you add Facebook login to your WordPress site, users will be able to login to WordPress through Facebook, and then login to Discourse via WordPress.