A recent post by Bungie revealed that, thanks to database programmer extraordinaire Roger Wolfson, games of Halo 2 online have 'officially surged over the four billion games played mark'. And that's in a little under two years after the game's launch.
Wolfson also notes that they're pretty close to hitting the four million unique players mark. Along with the update Bungie posted a lovely pie chart, which does more than take us back to our school days. It clearly shows that team-based games are by far the most popular.