PC gaming is very Popular. Its just that games on PC have 5-10 year shelf life with mods and stuff we can throw at em to upgrade em. So we play less games, but they are more fun to play than any console game. PC has been getting more games lately though with the rise of Mac in gaming PC gaming has seen a small spike in games.
Take HL 1 for example 8 Million sold or more. Still a grand number. HL 2 sold over 4-5 million retail and likely more through digital downloads and sales numbers. Valve and STEAM has never listed the total sales for the games though. Still lots that play online though. Not all under HL2, but very many under the many mods that HL 2 has and supports. Alot of digital sales are not counted in the sales numbers sites.
Sims series has sold or shipped over 100 Million. Its not hard to see how they have either with a new expansion every year or half year.
World of Warcraft has what 11-12 Million subscribers online.
STEAM has 15-16+ million users maybe closer to 20+ by now though 15-16 million was said a year or so ago and Xfire has 13-14 Million registered users.
Looks kinda popular. Still it isn't counting a large margin of people though. Some small slumps in PC gaming at times, but overall its still all very good. Every new game I play still has tons of players in it. Call of Duty 2, Jedi Academy, and Battlefield 2 still have lots of online players and lots of mods. All very fun. Like I said though PC games are built around having long shelf life. They are active for a very long time. At least the ones that are built with PC gaming in mind.
PC gaming is very popular today, not just here in the states but especially in places like South Korea, where the popularity of PC and online gaming has grown dramatically.