I try to be careful in my terminology. I purposely try to always use MMORPG when talking about wow, rom, etc…

I use MMO and MMOG interchangeably for any online game that allows more than one person to play, but excluding MMORPG, MMOFPS, etc..

Usually if the games a bit ambiguous or doesn’t label itself anything.

But that is history, not really current perspectives like you give.

http://en.wikipedia.org/wiki/Multi-User_Dungeon
http://en.wikipedia.org/wiki/History_of_massively_multiplayer_online_role-playing_games

As I’ve heard it, Raph Koster takes partial blame (or credit, perhaps) for coining the phrase “massively multiplayer”. This term was intended to excludes online games where the limit was 8-16 players on a single server, like most FPS games. Games like Meridian 59 counted as “massively multiplayer” because the servers could hold more than 16 players at once. (But, perspectives change and now that larger games host thousands of players at once, some people try to exclude smaller games from the MMO category; often these people try to show that the game they played first was the first “real” MMO.) Note that the first M is “Massively” not “massive”. “Massively” is a modifier to “Multiplayer”, indicating that there are a lot of people playing the game (with a cap higher than 16 or 32 or whatever). Size of the world has nothing to do with the definition.

You’re wrong that MMOs are a genre. Genre indicates either setting (fantasy) or gameplay (RPG). It might be better to think of MMOs as a medium, an alternative to “consoles” or “PC” or “handheld”. You could take a game concept and put it on different media, but the limitations and strengths of each one changes how the game would play. An RPG on the consoles (like popular Final Fantasy franchises) is different than one on a handheld system (consider the Final Fantasy Legend games on the Game Boy). An MMORPG is a different beast altogether, something people on here are probably familiar with.