Well before tf2 went free to play, cs 1.6 was the first and cs:s was the second most played game on steam. Actually since the main flood of people playing free tf2 is coming to an end I wouldn't be surprised if they are back at 1st and 2nd.
I'm really pretty surprised by all the Source Engine hate here (at polycount, specifically). I think it's pretty well known that Valve do a very thorough job of knowing their audience, down to what specs they can run. I am certain that the visuals of Source are restrained by the need to scale to a really, really wide…