Source
Turborilla:
"My friends don’t show up on the leaderboards"
If your friends or people you follow on Twitter don’t show up on the leaderboards you should go to the main menu, tap options (sprocket icon), tap account settings (globe icon, yellow button) and make sure that you’re connected to either Facebook or Twitter. If not, click the yellow connect button, follow the steps, and then go into the leaderboards again and check if you can see your friends there. You can back in and out of the leaderboard to make it refresh. Please try that a few times and see if your friends show up. We do not currently show your Google+ or GameCenter friends on the leaderboards, but they will be added in an update.