Very cool! Flatmin looks like an awesome service! Payback is definitely still feature-minimal :p Everything you suggested is actually a major item on my to-do list! Facebook integration has been brought up several times and I very much agree about the hex string scheme - the next thing I'm going to do is look into an invite by email/ join with referral link, etc. I have no intention of trying to monetize either - I made it purely as a learning experience for my own apartment, and it does seem like a difficult space to get into.