They could go along the same path as Foursquare and get as many users as possible and worry about monetization later. Although I think ianl is right, adding an intuitive mobile app with a killer UI is essential for your success/adoption. I would be down to use an app that had the activity list of TripAdvisor (with more unique user provided up to date options) integrated with the geo/social characteristics of Foursquare. I feel like you can’t underestimate people’s narcissism. If I am doing cool stuff all the time it would be great for people to be able to recognize it and make me, or someone cool (sticking with the Foursquare reference) "mayor" of awesome stuff to do. You could also look at the whole coupon based concept once you go mobile. Something like: "Awesomeness going on at location x come within the next hour and get ½ off amazingness!" I will keep an eye out for an app.

We'll build in the typical deal site affiliate stuff, do sponsored listings (make them float to the top) and let people sell experiences a la vayable.com or skyara.com.