[1]: http://store.steampowered.com/stats/
As of this posting there are ~52000 players currently playing CS according to Steam, with a peak in the past 24 hours at ~65000. For CS:S, the numbers are 38000 and 50000 respectively.
There are probably 20-100 times as many people playing Warcraft over time same time frame. It's about at the level of EVE online which is a vary nitch MMO. Distributed Systems Architect Jacky Mallett explained that “we hit a new peak of online usage, 63,170 users online concurrently” on January 23 http://www.pcgamer.com/2011/03/02/eve-online-has-over-360000...
PS: Considering the player base and ongoing subscription costs I would not be surprised if wow peak's at 2,000,000 - 3,000,000 players at the same time.