Noone "plays together".. People use Foursquare to broadcast where they are to the world, not play a silly game. Google Locations and Facebook already have this. Adding a gamified layer won't do much to attract users. On the other hand, putting a link in front of Google.com will.
(FWIW, the only two people I know who use foursquare actively compete to see who gets the most points every week.)