Reward for users

Dear All,

I am supposed to publish my first app this week.

As most of us are indie developers, we don’t have money to spend on conventional marketing methods and we should use our brains instead.

My first app is a Tournament like app in which users compete against each other, I will post its link in the forum as soon as I publish it in the market.

I have 2 questions here:

  1. Is it against any TOS, Google, Amazon and others, to consider a reward for the best score monthly?
  2. Is this going to be intriguing for the users and help the virality of the app?
  3. I have integrated the Facebook SDK into my app and users are able to use their Facebook account to login, I have heard that Facebook is not happy that game app share anything, such as users scores on their wall, is this true?
    I want to give them more score if they share on their walls?

Thanks in advance