Hello and welcome to the Auth0 Community!
When you click on “Preview” in the Universal Login settings, it should show you the social connections which you have enabled for your applications. However, if a social connection is configured, yet no applications are enabled for it, then the social connection will not be displayed in the login preview.
On the dashboard, when you go to Connections > Social, is there at least one application enabled for Facebook and LinkedIn?
Please let me know if this is not the issue!
Thanks,
Stephanie