How about we look at the 350+ TF2 players signed up for the i46 LAN, then... Outshining CS, BF, CoD LoL, Dota, or the category known as "other games". Looks like a couple American teams are likely to make it across the pond for the first time ever... If public servers, and f2p don't count, then perhaps the growth in the competitive community, and commitment of the players means something.
CS is of course incredibly popular, but TF2 has retained numbers, and is actually growing in certain ways.
I'd love to see some numbers on the population of those games over a number of years, but I can't seem to find the info.