In this case, it is expected that your realm has an identity provider with an alias `facebook`. If this provider doesn't exist the login form will be displayed.
The `kc_idp_hint` query parameter also allows the client to override the default identity provider if one is configured for the `Identity Provider Redirector` authenticator. The client can also disable the automatic redirecting by setting the `kc_idp_hint` query parameter to an empty value.