Foursquare is integrated into a lot of other products. For example Instagram and Path both use Foursquare to get nearby locations when uploading an image, and I'm sure there are numerous other services that do the same thing. There is still a tremendous amount of API usage even if it doesn't seem like such a huge "buzzword" app from a direct user point of view.

Looks like their path to sustainability is in their infrastructure and API and the Foursquare app is just a gimmick to keep users pumping in up to date location data.