Didn't they claim 15 million sales? If so, suckage all around. It could also means PC gamers have just abandoned it in greater numbers than the other platforms. I know I haven't touched in since early launch.
Again concurrent does not equal daily logins there could be hundreds of thousands logging on everyday but for 1 to 2 hour play session which not necessarily mean hundred thousand concurrent players.
Additionally, tons of people buy FPS games to play single player only. They never even go online.