I wonder if this sentiment is similar to the "no one uses Hotmail" type comments I hear from tech people. It seems like lots of people still use it based on the number I check-ins I see. It may not be quite as trendy these days but it's definitely gone mainstream.
Anecdotally, myself and several friends use Foursquare fanatically. Though I've been waiting forever for Foursquare to do something more with the service itself.
