The cumulative time devoted to creating Wikipedia, something like 100 million hours of human thought, is expended by Americans every weekend, just watching ads.
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In the 20th century, the bulk of that time was spent watching television, but our cognitive surplus is so enormous that diverting even a tiny fraction of time from consumption to participation can create enormous positive effects. Small excerpt from a great article about the change-promoting, intelligence-awakening tendencies of the internet.