Well, it's about that time for the people at Activision to brag about their sales figures again, not that they should be too happy after cancelling Guitar Hero/DJ Hero and closing down Bizarre Creations. But, Activision still has Call of Duty to fall back on (for now) and is reporting that Call of Duty: Black Ops is the best selling PS3 game of all time, using ChartTrack, NPD, and Gfk to report these numbers.
If these numbers are true, and they aren't just reporting units shipped instead of numbers actually sold (a common misleading technique) this would mean that Black Ops has beaten out other A-Rank titles such as Uncharted 2: Among Thieves (3.8 million), Metal Gear Solid 4 (4.75 million) and yes, even Infinity Ward's Call of Duty: Modern Warfare 2. Racking in $560 million within the first five days of its release easily contributed to these figures.
When it all comes down to it, the online experience speaks loudly. While Uncharted 2 and MGS4 are awesome titles in their own right, they cannot compete with the online experience of playing with friends (or strangers) online in Call of Duty. The second factor that also contributes to these sales numbers, is that Black Ops is much more accessible to the average gamer. In its core, Black Ops is a pick-up-and-play game. You don't have to be a fan of a deep layered plot or an amazing physics engine in order to enjoy a game that just asks for you to point gun and kill bad guy, and does so very easily and in a rewarding way.
While Halo Reach only released a couple of months before Black Ops, we have still yet to see Black Ops become the highest selling 360 game of all time. The 7th Call of Duty hasn't surpassed Halo 3's 8.1 unit million sales. Maybe Modern Warfare 3 will be able to beat out Black Ops record despite using the same game engine.
If you don't care about sales data, check out this fan-made live-action Modern Warfare trailer, which acts as a nice teaser for Modern Warfare 3.