At the time of writing, Steam has this to say:
Modern Warfare 3 34,081 online 34,700 peak Counter Strike: Source 39,915 online 39,947 peak Counter Strike 49,106 online 50,278 peak
It always makes me proud to be a PC gamer when I see genuinely great games like Counter Strike besting the juggernaut that the shooter for non-shooter players has become. Even when it's more than 7 years old now.
EDIT: Also, the new one's only in Beta and it's still getting pretty close:
Counter Strike: Global Offensive Beta 29,222 online 31,722 peak