The Facebook says that:
The access tokens are portable.
https://developers.facebook.com/docs/facebook-login/access-tokens#portabletokens
But! How can I use the access token, which was generated by an other app? If I create a session with that access token (with my app credentials) and make a /me request to graph API, then I get an other user id, than mine (which I got with my access token). So I cannot compare the two fb user ids with each other.
I know that the Facebook uses app scoped user ids, but what is the solution in this case?