Foursquare doesn’t break down country-specific numbers, but anecdotally speaking, it’s pretty big.
- Some months ago, I read in an interview (with the CEO, I think) that Tokyo is one of the cities with the most check-ins worldwide. (Sorry, can’t find it now)
- The API is now available in Japanese: http://tdoc.info/foursquare-API/ (doesn’t happen too often in the case of non-Japanese services)
- There are Foursquare user meetups, for example: http://www.meetup.com/foursquare… (which doesn’t happen too often in the case of non-Japanese services either)
Given that Japan is the world’s most advanced mobile nation and that location-based services and games had been around for years (laying the groundwork for the Foursquare concept), Foursquare’s success is not that surprising.