A defining characteristic of Web 2.0 is the bidirectional nature of communication. Rather than users simply reading and consuming content from websites, Web 2.0 is all about users being able to contribute back ratings, comments, and other content. During those early days of Web 2.0, my team built a system that depended on ratings, comments, and content contributions. I was pretty disappointed when I looked at the rates of contribution from users until I looked at the rest of the industry. Despite all the hype about this characteristic of Web 2.0, actual statistics for sites like Wikipedia, Digg, and Flickr were initially pretty disappointing too with contribution percentages with values that were less than one percent.
It took Facebook and Twitter to drive dramatic increases in individual contribut
ions. In fact, the primary actions users take in these systems are to contribute. I've polled friends on Facebook and people who follow me on Twitter and 
have developed the following model of contribution. People feel the most comfortable contributing on Facebook because they know it is only their friends and colleagues reading and viewing. Twitter is next but most people have friends and strangers too following them there so they are still reasonably comfortable contributing but less so than Facebook. Commenting on blogs, in comparison, is considered in third position feeling less personal and intimate.
If you have any thoughts on this and don't find this space too impersonal, feel free to contribute to the discussion using the commenting mechanism on this blog.