I'm sure the number varies widely depending on the site, but there is a generally agreed upon concept known as the 90-9-1 principle. From wikipedia :
In Internet culture, the 1% rule or the 90–9–1 principle (sometimes also presented as 89:10:1 ratio) reflects a theory that more people will lurk in a virtual community than will participate. This term is often used to refer to participation inequality in the context of the Internet.I think this is more of an orders-of-magnitude principle, and isn't really based on any specific data. Your mileage may vary.
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 (for example, for every person who posts on a forum, there are generally about ninety-nine other people viewing that forum but not posting).