http://www.brighthub.com/video-games...les/35992.aspx
NOTE: These numbers were updated as of February, 2011 but the last reliable data for some was a while ago (I'll put an estimate of the date next to each one).

Let’s take a look at the top MMO subscriber populations. Remember that these figures come from the developers or publishers and they all apply slightly different criteria to count active subscribers. Some of the figures are based on revenue and subscriber estimates. This article is only examining games so social MMOs like Second Life are not counted. These are rounded figures.

World of Warcraft – 12,000,000 (2011)
Aion - 3,400,000 (mid 2010)
Runescape – 1,300,000 (2009)
Lineage – 750,000 (2009)
Lineage II – 750,000 (2009)
Dofus – 520,000 (mid 2010)
Final Fantasy XI – 350,000 (mid 2010)
Eve Online – 325,000 (2011)
Lord of the Rings Online – 210,000 (mid 2010)
City of Heroes/Villains - 125,000 (2009)
Age of Conan – 120,000 (mid 2010)
Ultima Online - 100,000 (2009)
Everquest - 100,000 (mid 2010)
Warhammer Online – 80,000 (2010)