Looking for some input for a leader board / incentive system for a new iOS puzzle game app. What are your suggestions?

I'm in the latter stages of developing my first app for iOS. It's a puzzle game and I want to add in a leader board system to encourage players to play more levels and gain more points. You get awarded points for each level you complete and bonus points for the time you complete it in. We also have daily challenges where you gain more points if you keep coming back every day to play them so I want to incentivize them to do this with a leader board. I am struggling to find the best option for a leader board system though. Facebook only offers a "friends only" leader board option whereas Game Center only offers a global option. With the Facebook option, they may not have many of their friends playing our app so it will be quite empty as well as the fact that…