Angry Birds POP needs your permission to access your Facebook friends list in order to display friends in the game. If the permission is missing, your friends will not be displayed. To allow the game to show friends, navigate in your Facebook account settings to Apps > Angry Birds POP and remove the game from your account. After this, open the game and connect it to Facebook again. The game will now ask permissions again, so make sure that you allow the game to access the friends list before you accept them.