CS is still the most popular game on Steam[1] if you count both Source and 1.6, and has been for years. It is still played professionally[2], with a major tournament happening right now[3].

[1]: http://store.steampowered.com/stats/

[2]: http://en.wikipedia.org/wiki/Counter-Strike#Legacy

[3]: http://dreamhack.binarybeast.com/#CS16