According to Wikipedia:
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 term was coined by authors and bloggers Ben McConnell and Jackie Huba although there are earlier references to the same concept that did not use this name.
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. The actual percentage is likely to vary depending upon the subject matter. For example, if the forum requires content submissions as a condition of entry, the percentage of people who participate will probably be significantly higher than one percent but the content producers will still be a minority of users. This is validated in a study conducted by Michael Wu, who uses economics techniques to analyze the participation inequality across hundreds of communities segmented by industry, audience type, and community focus. This can be compared with the similar rules known to information science, such as the 80/20 rule known as the Pareto principle, that 20% of a group will produce 80% of the activity, however the activity may be defined.