No Friends scores in leaderboard
I'm assuming that you're still in the testing phase and haven't actually published your game via the Google Play Developer Console. Publishing is the key. There are two tiny sentences buried in a NOTE on this Google developer page:
Note: Social leaderboards will initially be empty until you publish the corresponding leaderboard by using the Google Play Developer Console
Social leaderboards won't be useful until after you publish. i.e. You'll never see social leaderboards during testing.