One of the big problems with starting a new service that relies on user submitted data is getting people to actually use it — nobody is going to routinely boot up your app if they don't have an incentive to do so. One way to tackle this problem is by working the service into a game, which is a technique that seems to be working quite well for Foursquare, a service that makes it easy to find your friends. GraffitiGeo is a new Y Combinator funded startup launching tonight that's looking to combine similar gameplay elements to take on a different space: restaurant reviews.At its core GraffitiGeo allows users to leave brief reviews of restaurants, or for those too lazy to do that, to simply leave a 'thumbs up' or 'thumbs down' — it's like a highly condensed version of Yelp, or a "Digg for the world".