Probably the most active piece of Nintendo 3DS software in Japan isn't Pokémon or the only-in-Japan-runaway-hit Monster Hunter. It's a preinstalled piece of software that creates a casual network with those around you, the Surechigai Mii Hiroba (Streetpass Mii Plaza) that makes use of the Mii Avatars users created when they registered.
Using it is simple. Two people who each carry a Nintendo 3DS come close to each other -- that's all that's necessary. No knowledge of networking needed. They can network when their 3DS is in "sleep" mode. It collects the avatars and some kinds of game data from the person you pass, and you can see the avatars in the Mii Plaza the next time you start playing.
Where it became interesting was during the recent Comiket when the number of people in a journalist's Mii Plaza topped 3000. Since the software isn't programmed for such large numbers, it stopped counting when it hit 3000 -- a situation that probably doesn't otherwise happen too often. There were certainly well over 3000 units "streetpassing" during the Comiket. The jouralists's Streetpass counter read 3229, but there were probably other units recording higher numbers.
What's the highest count you've gotten?