PlayOn has been collecting data for a while now, and Nicolas Ducheneaut posted a very interesting analysis on Terra Nova. It basically concludes that most players in WoW tend to spend most of their time alone, playing through the game as a single-player RPG set in a social space. It also has another very interesting stat, that 17% of all guilds seen in the game have only one member, on average they only have nine, and they are not typically very close groups.
Is this an aberration? I don't think so - my own play experiences tend to match up with the same stats. In any MMO, even when friends are online, I still tend to spend most of my time doing things solo. We all do our own thing, joined by a chat channel, and get together occasionally to do bigger things or when help is required.
What does this say of the current WoW design decisions though - the bulk of added content has been aimed at large, forced-grouping situations. If, as the article postulates, one of the major reasons that people pick up on and enjoy WoW is the solo-play in a social environment, these same players will be the ones that drop out when they reach the point where the content no longer supports that play style. Should MMO developers be focusing more on designs that require less direct socialization, instead of more?