Valve's project manager Erik Johnson, in an interview with Gamasutra, explained that he thinks "the analysis that every product needs to be a competitor in multiplayer, or an MMO, is incorrect."
"There are a lot of people who want an experience without the stress, so I don't see that changing," he continued. That isn't to say that single-player games are closed to innovation or shared experiences.
That could just mean that you want to be able to chat with other people who are playing through the same part of the game as you, or the fans can write commentary nodes in the game and everyone can experience those to take advantage of the fact that there is a huge community of people that want to interact with each other.
Valve has already put developer commentary nodes into its recent games beginning with Half-Life 2: Lost Coast. Portal 2 will be released in February for the PC, Mac, PS3, and Xbox 360.