One Internet folk wisdom is the 90-9-1 rule of participation on the Internet. This basic formula is that 90% of users will lurk, 9% will contribute occasionally, and 1% will account for most of the contributions.
Wikipedia notes that this rule is an "Internet theoretical concept." In a 2006 post, Jakob Nielsen calculates some actual statistics based on contributions to Amazon, Wikipedia, Technorati, etc. and suggests that the real numbers of participants does not easily map to the theoretical concept. For example, with blogs he notes that the actual percentages are: 95%, 5%, and .01%. In short, a very small percentage of users make most of the contributions on blogs.
A recent paper by Ochoa and Duval (2008), Quantitative Analysis of User-Generated Content on the Web looks at the actual participation rates of various websites including Amazon, Furl, Fan Fiction and others. There results indicate that there are different patterns of participation in these various sites. For example, 10% of the users contribute 50% of the Amazon reviews, 64% of the Library Thing book catalogues and 75% of the Merlot learning objects. Ochoa and Duval also note three general patterns of production-- fat-tail pattern with many users making contributions, a "fat-belly" pattern in which a smaller group of "star users" make a larger percentage of the contributions and a "prolific user" pattern in which one "mega-star" user makes an extraordinary percentage of the contributions (for example in Merlot, one user accounts for 12% of all the learning objects in this open source repository.
The overall point is that generating participation is a very difficult challenge and takes a substantial audience to achieve. Obtaining participation is a very high bar for Web 2.0 developers who are interested in engaging participants in content generation. Nielsen offers some useful ideas for effective engagement of users.