There have been a variety of figures bandied about for the ratio of content contributors to browsers / lurkers on social media sites.

From CNET News.com:

A recent Hitwise study indicates that as few as 4 percent of Internet users actually contribute to sites like YouTube and Flickr, and more than 55 percent are men. […] To be the mainstream trend (that it [Web 2.0] deserves to be), it must evolve from the currently small group of people who are creating and filtering our content to a position where the “everyman” is embraced.

and from vnunet.com:

Bill Tancer, general manager of Hitwise, said that the company’s data showed that only a tiny fraction of users contributed content to community media sites. Just 0.16% of YouTube users upload videos, and only 0.2% of Flickr users upload photos. Wikipedia returned a more reasonable percentage, with 4.6% of visitors actually editing and adding information.

Obviously, there’s some mismatch here, especially since Hitwise provided both sets of statistics, but I’d be interested in getting more figures if you have seen them elsewhere or have calculated some yourself.