Many have asked themselves why is Diablo III not on MMO site? I also wondered the same thing :3 It seems that the current idea is that the game will not be MMO... well kind of yes. Not Massivly Multiplayer yet multiplayer none the less!
In times past Diablo 2 was a rather huge multiplayer platform with parties up to 8 players. Blizzard seems to want to keep this to a more epic "play with your close friends" mode of about 4 players per map. I say about because the game still is in development and they are still testing optimal party size.
Lead Developer Jay Wilson was recently asked about the importance of Multipalyer during Blizzcon 2010 and he responded with an eloquent statemetn:
"For us it's a little bit more focused on the singleplayer, it's not quite the same as StarCraf