Or, the people who talk about MMOs and how massive they are in terms of players. I'm sure there are millions of people who play cards as well. Solitaire, even, which is played... when solitary... but millions of people play it, at the same time, with their own copy of the cards, and although they don't meet each other when playing it and they don't play against each other, they might read about it and chat about it online.
Some people describe MMOs as this amazing event in our history because of the userbase and how the users all play together. But we know the players don't all play together. In Second Life, you can have 50 or fewer, max, to a server. And you probably aren't interacting with all 49 of them. In a more traditional MMO, say, WoW or EQ, they have many "servers" (different from an SL server), so the massive tens/hundreds of thousands or millions of users will never, ever, interact in the game. Ever. Period. Online on a chat site or the forums, sure. Grouping and raids you will have a bunch of people, and I am not sure of the exact numbers for the different games with their different engines (and PvE versus PvP, perhaps).
Overall this is not too much different from online games from the Call of Duty and Halo series. A large number of people, playing together. The "world" may not be "persistent" in the same way as in an MMO, but it is the "same" in that it is the same map or selection from a set of maps (like selection of certain areas from a larger game world, which these maps often are).
I don't have the exact numbers on these FPS games either (I'll work on that), but these FPS games and the MMOs all have huge numbers of players and the numbers of players that interact is probably about the same, both in-game and outside of the game. If you only focus on the interactions in the game (and there are a lot of ways for this type of interaction) then you are missing out on a lot of important interaction and information sharing that goes on through other channels.
Even with games that were purely single player, like Myst and Adventure (on the old Atari 2600), there were thousands of people playing it and then interacting through other channels. I found the easter egg of the programmer's name in Adventure, but I certainly didn't figure out how to get the invisible dot and where to put it all by myself. A friend of mine told me how to do it. How did he know? Someone told him, or he read it in a magazine. We can modernize that sentence somewhat, "someone emailed him, or he read it online." Same basic human communicative processes.
MMOs are not quantitatively different from what we have already experienced.
