Well, first, it's important to note that actually not many people contribute content on the internet. In fact, research shows that in many services most of the content is contributed by a small percentage of the users. 

Check out Jakob Nielsen's Participation Inequality: Encouraging More Users to Contribute post, which provides more data on this 90-9-1 % user distribution behavior. 

Two more additional interesting readings on this fascinating topic:

Why Do People Contribute to Digg or Wikipedia? by Diane Ensey

Why do people contribute? (who is dirtyfratboy?) by Jason Calacanis