The game Diablo 3 is an odd product. In many respects, it is a single player game, yet it requires a constant Internet connection (which led to total disaster at launch), has an auction house which includes all players (I think, lots of players at least), and has some sort of in-game general chat. (It's not clear to me if this is all players, or if there are servers like in an MMO but they aren't highlighting the servers.)
It's not an MMO, in terms of the massively, since the maximum group size is 4. Yet, everything else about it is like an MMO, given the chat, the auction, and the focus on the end-game and leveling up. Basically, it's like an always-instanced MMO, with a maximum group size of 4.
Perhaps it's a feeder for WoW, as if they need more players.