I just came across this article   http://www.gamesindustry.biz/articles/13-million-xbox-live-players-on-call-of-duty-4

'There are currently 13 million unique Xbox Live users playing Call of Duty 4: Modern Warfare, according to a new interview with Infinity Ward community manager Robert Bowling.  Bowling commented that: "Call of Duty 4 on Xbox Live just exploded. I mean right now we're at 13 million unique users on 360 and that's insane.'

I know as from Nov 08 CoD4 sold over 7 million copies on the 360, But I suppose millions of users bought used copies, rented or borrowed a copy of CoD4 from a friend.  But still thats is a insane number.  While he states the PS3 numbers were around '4,381,276 unique users in January. With a significant percentage of users likely never to have played the game online'

CoD4 is way up there with games such as Counter Strike, WoW, Halo 3 etc.....

I think this is old. But anyway, interesting to see how CoD 4 is used by multiple people on each console, on average just under 2. That 13m is offline for Xbox and PS3's 4m is only online.... which might mean some undertracking, but there is way too much guesswork here to prove anything. (Generally the number of people playing online is lower than the sales {Gears of War 2, Killzone 2 and probably more})

interesting

He says that significant percentage of the 4,381,276 users on the PS3 likely have never to have played the game online.

The only thing missing now is the PC numbers sold and PC playes online.  Saying that, CoD4 is still in Steam's top 10 sellers and as been for a very long time. 

@ canch, I was not sure whether he meant playing the game with the PS3 connected to the internet, or people playing a deathmatch etc online

Reguarding Xbox he said that 13 million have tried MultiPlayer

Unique users can only be determined via an internet connection. Those numbers do not include offline players for either the PS3 or 360.