Lot of image hosting websites rely on AdSense to pay the bandwidth bills. Similarly, there are YouTube clones that aggregate web videos and monetize page-views through Google AdSense. Then we have thousands of personal blogs that are share photo galleries and web videos but have little or no text content.
Some of these websites could be doing really good with regards to AdSense revenue but Google also gives Adwords advertisers an option to block display of ads on web pages that contain only "user generated content".
Google advertisers can also prevent their ads from getting displayed on social networks and online forums where "content changes often and is harder to monitor than content in more static pages."
And the recent changes in AdSense Policies allow publishers to place AdSense unites on registration form and pages that return 404 errors but again, Adwords clients have an option to hide their ads on such error pages.