According to
Major Nelson CoD4 is already the fourth most popular title in terms of Live activity (which counts people playing while connected, not just people playing online), behind only the juggernauts of Halo 3, Gears of War and Guitar Hero III. It will be interesting to see how well it performs in the long term but if the multiplayer is as solid as everybody says there's no reason why it couldn't stay popular and maybe even go up a place. Probably not above Gears because that was pretty much the constant number one until Halo 3 finally usurped it, but then when UT3 hits it will be interesting to see how that shakes things up. 360 owners sure do like their online shooters...