Here is a link to a site that has a pie chart showing marketshare for MMORPG's. The data has not been updated since june, so it doesnt reflect World Of Warcraft taking the top spot since they opened subscriptions to China and Singapore. Last count from blizzard is that they hit 3.5 million subscribers after making WoW available in China. Pretty incredible.
My suprise is that lineage 1 and 2 take up about 40% of the market. I havent heard much about it, let alone that it was that huge.
Its also good to see that Ultima Online has a decent share of the 2ndary market, seeing as how that game came out when i was in highschool. Incredible.