Foursquare is still the UK’s most popular location based check-in game to play on your mobile. I’ve said it before that Foursquare would come out on tops over Gowalla and Brightkite.
These maps (thanks to Checkin Mania) indicate the difference in the number of locations that users have submitted to each service. The growth from crowd sourced locations and points of interest reflects in the growth of that services usage (ie the most popular service gets the most contributions from users) The point of reference is London Borough Market - one of the more famous places in London to visit.
Aside from this, a fairly random survey of people using these ‘games’ showed a considerable higher interest in Foursquare then Gowalla. Gowalla was growing in awareness, but people I’ve asked complained it was slower than they expected.
Finally - if you’re getting started in social media marketing - don’t spend your money on a campaign only for one social network - spread it around so that everyone can join the game.