When we last checked in with Tobold, he was just starting up a new blog series looking into why we play MMO games to a greater degree than single-player games. That isn't to say that the MMO genre is bigger than the single-player genre, just that MMO gamers tend to focus on massively multi-player games more than single-player games. His first two articles examined Storytelling and Gameplay elements in MMOs and since then he has done pieces on Challenge, Character Development, Rewards, Social Interactions, and Learning.
The series just wrapped up and Tobold wrote a nice summary of why he thinks we mostly choose MMOs over single-player games. He feels that while we may play single-player games that have strong elements of story, gameplay, or challenge, the social aspects of MMOs seem to be the trump card. MMOs can have many weaker core elements but social interaction (direct) or simply participating in a persistent game world (indirect) appears to make up for those deficiencies. It's a long series to get through but well worth the read if you're into these kinds of high-thinking philosophical discussions.