I think currently a few of my friends are playing for free and their scores don't show on the daily challenge. I believe unless you pay, you are unable to see or be seen amongst your friends. I think this is a big mistake, because whats so fun about the daily challenge is competing with your friends. When free users dont see that aspect of the game, theres no motivation to pay and get better to beat your friends or be more competitive. Ive noticed my friends that are playing for free have dropped off and dont play as much because its not very engaging when your by yourself in the game.