Activision revealed some rather impressive Call of Duty related numbers in their latest financial report. Call of Duty: Elite now sports 10 million users, and two million of these are paying subscribers.
Perhaps what's most impressive is that 40 million people play some version of Call of Duty every month (unknown whether some of these play multiple titles, but that sounds likely). Battlefield 3 on the other hand has 6.2 million monthly players.