Matt Hodgson had some interesting thoughts on Meeting needs - why social computing works. He wrote in reaction to another blog posting, which nearly illustrates the point.
Those needs relate to belonging to a community, to gain esteem through the respect of others and to self-actualize. All of these are at or near the top of Maslow's Hierarchy - those needs we seek after the basic needs have been met.
Matt further breaks out these needs into those met by social computing in general (community; esteem through respect) and that met by blogging specifically (self-actualization). It is clear to me in these discussions about "who participates" or "why do they participate" that these needs are met to varying extents in varying ways for people. This may sound like a no-brainer comment, but the people who have these needs met in social software do not necessarily represent the majority. I think evidence from Pew Internet and other research suggests that more-and-more people are finding value in this medium though. And the technologies are fitting into how people want / can get their needs met. This could almost be a chicken-and-egg discussion: are people becoming more comfortable with the tools, or are the technologies supporting people better?
This line of thinking is helpful in the larger discussion of why social software and social media continue to be interesting. These needs relate to individuals. There are also the needs of the community, the needs of the businesses participating, etc.