Diablo III logs around 1 million unique players per day, former game director Jay Wilson revealed today during a Game Developers Conference panel titled "Shout at the Devil: The Making of Diablo III."
Wilson also noted that over 3 million unique gamers play Diablo III every month. After spending six years on Diablo III, Wilson announced in January that he is transferring to a new team within Blizzard Entertainment.
As of December 31, Diablo III has sold in (that is, shipped) 12 million copies.
Also during his talk today, Wilson admitted that Diablo III's auction houses--both real-money and in-game--have negatively impacted the game. Though 50 percent of all players make use of the auction houses, Wilson explained that making real-world money has become more of a motivating factor for players than killing Diablo. He said the team at Blizzard is working to remedy the situation, though nothing specific was mentioned.
For more on Diablo III, check out GameSpot's review.