Currently not much, that will be one of the challenges. Later on – gamification, how well are you at predicting upcoming trends in certain cities/categories.
Yeah, that would be my first thought as well. It seems like a challenge to attach users to something like this when it exists outside of an application they already use (eg. Facebook et al). I would be interested in an update down the line to see if you are able to capture regular usage and how you did it.
You'd probably have to limit the allowed number of "guesses" per day/week. But even then there's a risk of people trying to game the system (creating multiple accounts etc)