I would disagree - I think most FPS and even a lot of games marketed as massively multiplayer games are just multiplayer games. I would classify games as falling into one of three categories:

Single player (self explanatory)
Multiplayer - games where the the number of participants in the game is equal to or less then Dunbar's number
Massively Multiplayer - games where the the number of participants in the game is equal to or less then Dunbar's number

I would define a game as being a persistent space/activity where the participants interact. By these definitions some examples outside of computer gaming would be:

Single player: reading a book
Multiplayer: baseball
Massive Multiplayer: World War II

The number of participants in the game greatly changes its nature. In a single player game the sole purpose of the game is to meet the needs of the single participant. In a multiplayer game you will not see units of cooperation exceed any participant's known social contacts - i.e. everyone knows their own teammates. In a massive multiplayer game units of cooperation often exceed any single participants known social contacts and gives rise to the concept of nations, and coalitions of nations.

To me (and many others) it is this ability of massive multiplayer to simulate large social structures that makes it unique. With large social structures you get things like heros/villains, causes, legends - wars.

Most FPS games I would say are simply multiplayer games. To be fair the number of participants in a game does not have to be determined by how many can interact simultaneously. One could envision a turn based game that only allowed one player to interact at any instant and yet still support thousands of participants. So too could an FPS that supports 48 players per match create a larger meta game by linking those matches together in an online gaming league. Similarly Dust 514 could create a massive multiplayer game by linking the results of matches to the larger persistent EVE universe.

But I think the litmus test would be if the participants every felt like they were part of a group larger then the one they created. Does a Dust 514 player feel like he is participating in a greater online war? Or does he just feel like he is participating in a series of unrelated battles? If it is the former then it is a massive multiplayer game - if it is the latter then it is just a multiplayer game.

I personally think that although there is some basis for arguing that Dust 514 is a massive multiplayer game based on the current set of features it is still closer to just being a multiplayer game (although a pretty good one). The ground work is there to make it massive and lets hope that it is steadily pushing in that direction.