You can add non-Facebook friends to your leaderboard via our friend's code feature.  Once added - you will see their scores on your leaderboard and can earn rewards for beating them.

You can also give your code to others to appear on their leaderboards

If you enter a friend’s code, you can see their scores, but they won’t see yours unless they enter your code (or unless you’re Facebook friends)

You can add friend's codes and find your own code via the Friend's code button in the settings menu.  Please note - you must be logged in to Facebook to use friend codes