Great to hear that it's working so well for you. It's taken a bit of effort to get here, and can't wait till we can get a release out where everyone says the same thing! (We're close we think).
As for invitations limits, the below article explains it best. I don't think there's been any updates to this policy since then. To sum it up, it's a dynamic amount depending on your application and is based on how many invitations are normally sent and the ratio with which they are accepted. So if you're users send them to their friends and there is a high acceptance rate, the rate (should) be increased with time... I have no knowledge of how this works in practice as I haven't tracked invitation limits in the past.
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developers.facebook.com/news.php?blog=1&story=83 
Limit on Invitations
