"Call of Duty 4 has racked up 11 million unique players since launch on Xbox Live, and over 4 million on PlayStation Network, according to Infinity Ward's Rob Bowling."
http://www.eurogamer.net/articles/cod4-has-11m-live-4m-psn-players
That's only the people that have connected their 360s/PS3s to the internet. According to VGChartz, it has sold 7 million on the 360 and 4 million on the PS3.
So, it's around 5 million undertracked then?