The Wrath of the Lich King, Blizzard's recently-released expansion pack for the game, has sold more than 4 million copies since its release on November 13, 2008. 2.8 million of those were sold in the first 24 hours of availability, according to Blizzard.

World of Warcraft has been available for the Mac and PC since 2004, and has spawned two expansion packs. The game--played entirely online--requires players to pay a monthly subscription fee in order to play. In October, Blizzard that it had hit the 11 million subscriber mark.

The last time Macworld reported on World of Warcraft subscriber statistics, forum commenters voiced some questions about how that number is tabulated. So without revision, here is Blizzard's definition:

"World of Warcraft subscribers include individuals who have paid a subscription fee or have an active prepaid card to play World of Warcraft, as well as those who have purchased the game and are within their free month of access. Internet Game Room players who have accessed the game over the last thirty days are also counted as subscribers. The above definition excludes all players under free promotional subscriptions, expired or cancelled subscriptions, and expired prepaid cards. Subscribers in licensees' territories are defined along the same rules."