FOSFacebookBundle and FOSUserBundle working together
i recently worked on a project in which i included these two bundles. A user provider is just a class that is responsible for checking auth data. Let's say you have a shop and customers should be able to login, so you have to define a user provider that handles customer entities that are stored in a local database.
But in your case you have Facebook Users that should be able to login. So you define a user provider that can handle these type of users (with the help of the FosFacebookBundle).
It's really simpel. See "Example Custom User Provider using the FOS\UserBundle" chapter further down. You can just copy and paste it into your own bundle.