It’s called the 1% Rule. It has its own wikipedia article. And getting a wikipedia article is more difficult than you might think. So it must be true.
“The 1% rule states that the number of people who create content on the internet represents approximately 1% (or less) of the people actually viewing that content (e.g., For every one person who posts on a forum, there are at least ninety-nine other people viewing that forum but not posting).
The “90-9-1″ version of this rule states that 1% of people create content, 9% edit or modify that content, and 90% view the content without contributing.”
That explains it.