I enjoyed Caroline McCarthy's questions about Foursquare's sustainability.
But let's not limit the definition of success to the Foursquare product as it stands-the API is where the real action is.
Venue and advertiser innovation will drive the platform's success, as with Twitter. Businesses and marketers can leverage the Foursquare platform to create engaging experiences wherever people gather, and they don't need to wait for Foursquare to make this a default behavior.
Marketers are dipping their toes in to the water of location-based social networking, using the game elements of Foursquare to enhance their marketing programs, but few are creating brand-relavent and ongoing experiences that add value for the user.
As users tune out broadcast messages, marketers will turn instead to memorable, high-ROI experiences in the real world that leverage Foursquare, or Yelp/Facebook checkins, or Gowalla, or the application we are all going to be using tomorrow. Each of these is just another doorway to reach today's hyperconnected audience- and all their Foursquare friends.