Online multiplayer FPSes such as Counter-Strike don't exactly count as a MMO. They are multiplayer, yes, however they do not classify as massive since they're limited (in comparison) to the number of players.
Diablo 2 is also not technically considered a MMO because, while it does have a large playerbase and a level of persistence, it is highly instanced in the maximum number of players that unfortunately do not exist in a persistent active world.
Then you ask... what about Guild Wars?
I'm trying not to mince words too much on this, as some may call on the wording especially with companies these days blurring the definition with their games, however the general model of a MMO is that not only is the world and its characters persistent and active, but the game involves a large number of players simultaneously playing with each other in that world.
I can tell you though that CS is definitely not a MMO.
