When someone sent me friend request, I check his achievements to see what level fan he is.
(yes, usually playing the game from the old pc version where I can see that).

Also I 'm checking in the end of the season some friends-supporters (?) and "clean" my list.
Most of my friends-supporters are common friends with my other friends.
I saw them support in many games, they saw me also that I am a loyal supporter and accepted my friendship.

But don't expect high %. I can say that from about 100 (TE and fb) friends, I have about 10-20 supporters in every game.

of course I have some fb friends for fun and not supporting.