According to Joystiq, earlier this year, Xbox Live had ~20M users, 10M of which were admittedly non-gamers, and used the device for Facebook, Last.fm, Twitter, etc.
They (MSFT) stated they had an online peak of 2.2M concurrent users.
According to Steam Online Statistics (http://store.steampowered.com/stats/), there are currently over 2.5M users online at the moment.
Granted this does not explicitly mean these are all FPS players, but Steam is just one of many PC gaming services, and its users outnumber Xbox Live all on its own. These numbers don't include Battle.net 2.0 (no FPS, but PC gamers nonetheless), D2D, StarDock, etc. Then add all the non-tethered games, and you've got a community that probably outnumbers Xbox Live's entire footprint, even including the irrelevant non-gaming users.
The PC gaming community is alive and well, thank you very much. You can keep your antiquated hardware and AA-less games. We'll keep ours.