If you have played any Facebook games and wondered why you do not see some of your friends (even if they played the game at some point, this could be Facebook limiting your seen friends on their end.
Facebook only checks up to your first 1000 friends on your account. This means if you are not seeing all your friends in game, it could be that they are simply marked as above that number and thus would not be available in that game. This may also includes any friends that play or have played our games, as any one above 1000 would still not be counted towards your in-game friends list.
Unfortunately, there is nothing we can do for this issue, as it is entirely on Facebook's side and thus out of our control.