According to the folks at Bungie, (and really, they oughtta know), as of June 5, about a half-billion games of Halo 2 have been played on Xbox Live. Half. A. Billion. Says Bungie:

To clarify - that's actual games, not player-sessions. That also doesn't include games we lost because the website was down, or through other technical issues.

All I can say is...you folks do know that there are other games out there....right?