Apparently, almost half of the gamers with an account on Xbox Live don't pay for a Gold membership (which would give them access to online multiplayer). This makes me even more certain that online play and multiplayer are not, as they are sometimes touted to be, the future of gaming. They are merely options, with offline play and single-player games remaining very popular.
We can assume that many 360 gamers want to play online but either will not / cannot pay for it or they don't have enough free time to justify the expense. However, we can also assume that many gamers pay for a Gold account but don't regularly use it for online play (I'm an example). I expect those and other factors make the Gold/Silver accounts balance a less-than-perfect indicator of interest in single-player gameplay vs multiplayer, but it might be roughly accurate in that regard.
More importantly, though, we can look at the sales figures for single-player games. Bioshock, Assassin's Creed, Oblivion, Fallout 3, Super Mario Galaxy, Wii Play, Wii Sports, Wii Fit, and Metal Gear Solid 4 are all bestselling games that thrive on offline, single-player experiences. Other hits like The Sims, Spore, and Little Big Planet are essentially offline, single-player experiences in which online components work backstage. Single-player games are still very profitable and still among the most popular games around.
And yet, how much money is there to be had from downloadable content? That potential will skew the market toward online gameplay. Single-player games will always have a large share of the market, but I expect we'll see more and more games using the model of Oblivion and GTA IV: single-player, but with DLC that does not always appear simultaneously as hard copies.