You can see that for example Fifa 11 is missing on that list and also Blops Zombies, at least on my and my friends list nearly no one plays that anymore... Here and there a death match but if you wouldn`t have MW3 you would be a lonely gamer...Me and my friend enjoysed Blops on PS3 even with MW3 already released but after 2 months nearly noone plays it anymore online (aside of random matches)...
For Xbox Live at least, plenty of people still play almost every CoD. I see people on my friend's list still playing MW2.
EDIT: For reference, here's the latest Top 20 Xbox Live games (via http://majornelson.com/)
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No one is forced to buy anything but I think it is not deniable, that for someone who doesn`t want to upgrade to the newer game of the franchise the new release is definitely a disadvantage for his enjoyment!