You might get different answers depending on the platform each person has played them on. Which platform are you interested in?
I have Cod 2,3,4,5 on 360 and Cod 6 and 7 on PS3 so PS3
I don't know if this helps but I think MW2 has more people on the 360 right now and Black Ops has more people on the PS3
I highly doubt that.
Last time I checked the 360 version had about 438k and the PS3 version had almost 600k. And they were in the same time period. I checked at my friends house there was only a hlaf hour difference around 4pm Saturday
BTW I live in Canada