It will probably be cool if you let users be able to customize the reward for inviting folks. For example, if I'm launching an app, it doesn't matter if you're number 1 on list or 1,000. Everyone will still be downloading at the same time when the app is released. Perhaps some users will want to customize such that when you invite 20 friends, you get x, when they sign up, you get y.
Good concept though.