This is the famous Yahoo! Pyramid that represents "phases of value creation" at Yahoo! Groups as outlined a year ago by Bradley Horowitz, the company's head of technology development.
A new study by Hitwise apparently suggests that the number of generators of user-generated content is even smaller:
"A tiny 0.16 percent of visits to Google's top video-sharing site, YouTube, are by users seeking to upload video for others to watch, according to a study of online surfing data by Bill Tancer, an analyst with Web audience measurement firm Hitwise. Similarly, only two-tenths of one percent of visits to Flickr, a popular photo-editing site owned by Yahoo Inc., are to upload new photos, the Hitwise study found."
-- reuters via psfk
One caveat: that's percentage of visits (uniques?), not percentage of users. Anyway, to shamelessly quote myself, "the bigger force is not consumer-generated content, it's consumer-edited content."