I get the following error, trying to import:
"Module '".../node_modules/angularfire2/auth"' has no exported member 'FirebaseAuthState'."

And this one, trying a different path:
Module '".../node_modules/angularfire2/index"' has no exported member 'FirebaseAuthState'.

EDIT: I'm trying to use this function from the docs:

  signInWithFacebook(): firebase.Promise<FirebaseAuthState> {
    return this.afAuth.auth.signInWithPopup(new firebase.auth.FacebookAuthProvider());
  }

(https://github.com/angular/angularfire2/blob/master/docs/Auth-with-Ionic2.md)

I use the latest version (4.0.0-rc.0), in an Ionic app (I'm using the guide in the docs).