In a rather recent article, Tim Harford improves his answer to one of his readef's questions : Why does anyone bother contributing to Wikipedia ?
Harford last answer is : people post videos on Youtube because they really want you to watch them. He says this runs counter to the usual economic thinking about public good provisions since people are less likely to bring anything into a community if the free rider problem is too great. Hence you are less likely to take an expensive bottle of wine with you to a party where you're not likely to get a sip than to one where you're sure you'll get a glass.
Only the logic is entirely different in Wikipedia's case. The reason why is because we are sharing non-rival goods. In a way, adding an article to Wikipedia or a video to Youtube is often, where we're not after fame, equivalent to enriching our own encyclopedia or our own video library.