If I were Foursquare I'd admit defeat, drop Swarm, do a big push to relaunch Foursquare with a new app (if nothing else just to get people interested again) and think really hard about a killer feature to lure people back into the app.

One aspect that's really underplayed I think they could go with as a complement is diary and life tracking. It could be very interesting to have as an experience where you could relive where you've been and done.