I think everyone is making too large of a distinction. Just because games have multiplayer modes in them does not make them an essentially multiplayer game. I think it gets divided up a little bit more than that, to be frank.
First off, to clarify, let's look at wholly multiplayer games. MMO's are, by definition, multiplayer games, so they dominate this category, but are not the only games here. Stuff like World of Warcraft, EVE Online, City of Heroes, Team Fortress 2, and the like are all games that were designed from the bottom up to be played with other people, with absolutely no single-player mechanics anywhere to be found.
Then you have games that have a single-player aspect, but the multiplayer is the main promotion. Stuff like Resident Evil 5, Army of Two, Halo 3, Gears of War 2, Left 4 Dead, LittleBigPlanet, Demigod, Sins of a Solar Empire, all sports games, and so forth.
Then you have games that have single-player campaigns with an equal share in multiplayer gameplay. Games like Call of Duty (yes, even Modern Warfare), the original Gears of War, the original Halo, Killzone 2, F.E.A.R., Command and Conquer 3, and generally most first-person shooters. You know, games that have a compelling single-player campaign that is enjoyable to play through, but also have multiplayer modes that people enjoy to play.
And then, yes, we have the wholly single-player games, such as inFamous, BioShock, God of War, Ghostbusters, Fallout 3, Prince of Persia, Uncharted, Dead Space, Assassin's Creed, Final Fantasy, Mass Effect, Half-Life 2, Portal, Shadow of the Colossus; the list goes on and on. All games which don't have any multiplayer functionality at all, and all of them powerhouse games with a very large number of sales. Hell, even games with extremely limited multiplayer functionality, such as Metal Gear Solid 4 (with Metal Gear Online) and Grand Theft Auto 4 sell better than games that tout their multiplayer as big selling points.
In fact, I daresay, with the exception of companies like Blizzard and Turbine, Epic and Bungie, most game developers these days bank their profits on single-player games. Sure, some developers look to be expanding out into the multiplayer scene, such as BioWare with The Old Republic, Valve with Team Fortress 2 and Left 4 Dead, Square Enix with Final Fantasy XIV, and so forth. But that doesn't mean that they're going to kill off the existence of good single-player games. All three of those developers are known for their amazing single-player products, after all, and they still promise to have more of that sort of product coming out in the foreseeable future. The industry is expanding, is all, growing in all directions, which means that developers are putting out new products to keep up with the growth. It doesn't mean that they are abandoning something that is so integral to the community of gaming.
Truth be told, my problem with games these days is that not enough of them have multiplayer. I look through my collection of games, looking for a good multiplayer game, and have severe difficulty finding one that I actually have any desire to play. Sure, a lot of FPSes are cashing in on the online gaming scene, but I've always been a fan of couch co-op, or at least same-room multiplayer. It's always more fun to game with others when you know who they are and can have a conversation on the side, which is a trend that has steadily died over the last few years. And I think that's the bigger crime.
Long story short, though, the single-player game industry isn't in any threat of being killed off. It's thriving as powerfully as always, and keeping the pace with this new outburst of MMO's and what-not in terms of collective sales. I can't give any numbers to back this fact, of course, and World of Warcraft is a monster in terms of sales, but the fact that we still have so many quality single-player games is evidence enough for me to feel confident in that statement.