Cross-platform Leaderboards

Hi Devs,

I'm planing to develop an endless runner 2D game for android, iOS and Facebook canvas. The game is going to be simple. Player needs to collect as many fruits as possible and collect points. One weekly basis I'm going to reward the top three players with physical rewards to engage the audience and attract more players.

But before I begin, I need some suggestions from you:

1. Which hosting service would you prefer for facebook. The game isn't going to be that intense but I expect a minimum of 50,000 monthly users?
2. Is there any asset in asset store to provide cross-platform leaderboards so I can see which are the top three players and reward them?
3. Authenticate users with facebook so when players come to claim their rewards, I can verify them.

Thanks for your time .