Source tends to top it, and thats 1) Only including Steam calculations, and 2)not counting any console shooters (which like it or not is where the large majority of people play shooters these days)
Past the fact that overall it falls far short of the most played FPS if you count all online fps outlets, it hasn't done anything new for around 6 years, it simply isn't relevant anymore. I get it, it was a great game, I played in CAL back in the day and dropped out shortly after source reared its ugly head, but seriously let go of it
i <3 CS 1.6.