As everybody knows Wikipedia is an online encyclopedia, launched on the web in 2001, available in over 280 languages and with more than 35 million entries. Every page can link you to many others making your research way easier than using the books. But what does it have to do with the audience participation? Well Wikipedia basically exists thanks to its users. Users can donate money in order to support the website. They can also add notes and parts to the already existing pages; they can also add pages themselves. Obviously everything that is written down form the users will be double checked from the experts first and if they agree that it is right and correct, it will be published then. I personally find this website one of the most important talking about the audience participation in something online not only because it has EVERYTHING inside (from the recipe of a typical dish, to the most famous actor, ending with the explanation of one particular event) but also because it is very well done.
Fun fact: no matter what page of the encyclopedia you are in, you can get to the Adolf Hitler page in less than seven clicks. Try starting from here for example.