Where does all this stuff come from? According to this interesting article (also linked above) it comes from a cognitive surplus in society. The interesting thing is that he says that there have been other civic surpluses. Initially society didn't know what to do with them and for a time wasted them. But then people found out how to put these surpluses to good use for everybody.
With the Internet, there is the capability to use excess cognitive energy to benefit a wide swath of people. People create media and literature to share. Because the Internet collapses distances, there are more opportunities to discuss common interests with other people. Projects such as Wikipedia and MIT OpenCourseWare provide aggregated human knowledge at no cost to the user.
Early in my life, I could only participate in consumption of mass media. Now we all
"just assume that media includes consuming, producing and sharing".
I like that idea better.