I wouldn't say you were totally wrong -- the core checkin model hasn't changed, but it's just like twitter (as you point out) where the use case and userbase is what evolves and eventually hits critical mass.

That kind of thing is why I like to do pure-tech startups vs. social startups; it's a lot harder to predict success with the consumer things. When they do take off, though, they turn into Facebook -- and Foursquare definitely has done well for itself ($600mm valuation last year, so presumably over a billion now, a few years after founding) on the business metrics, and seems to have good user metrics too.