Nexon have set a new world record for concurrent users with Dungeon Fighter Online. The game that is called Dungeon&Fighter in China, recently had more than 3 million concurrent users in mainland China. The game has held the world record since reaching 2.6 million concurrent users in June and saw a massive increase in players thanks to a content pack released this summer. The game has been available on the Chinese market since 2008.
Naturally it is difficult to gage these kind of records, with the likes of Call of Duty: Modern Warfare 3 being available across three platforms for comparison, nonetheless it's difficult to see 3 million players playing any other game at the same time.