PC fanbase for CoD: MW3 and Blops 2 was actually pretty decent based on steam stats/player counts until AW came out and finally broke things three ways. MW3 has surprisingly had real staying power. Even MW2 outlasted Blops. World at War has a decent user base which is weird. Over the last thirty days AW+Blops2+MW3+MW2 = 12,000 players + Waw + Blops + Ghosts = 15,000. Which if it was one game would put it up in the top twenty I think. It has definitely collapsed though since it used to be like 30,000 for newest and 20,000 for last game, so has CS and CS:S and TF2. I remember when the top ten on Steam was like TF2, CS:S, CS, Garry's Mod, L4D, the latest two Call of Dutys and newest single player games. Now it's Dota 2, CS:GO, TF2, Civ V, Warframe, Skyrim, Garry's Mod, Pillars of Eternity, newest Football Manager and Arma 3. And only like two of the spots ever change.
Ghosts hilariously cratered almost instantly, which is why I think Activision ran like three free weekends.
Though a lot of the modes are dead except for like an hour each day.