UGS Leaderboard Facebook friends only

Hi, i would like to have a facebook friend leaderboard on my mobile game.

I thought i could authenticate to ugsLeaderboards with FB and then retrieve my playing friends’s score. like this Facebook Login and Leaderboards - Developer Guide
But Leaderboards does not support anything like that (even if its predecessor did).
Can i use fb app scoped id as ugsAuthentification ID, this way both fb and leaderboards id would match and i could use normal GetScoresByPlayerIds() function.

Am i missing something ? Do i really have to set up a custom database to match ugsId and FbId? Doesnt sound good.