The Wall Street Journal has a story on how a new ad platform is gaining in popularity that basically seems to cover up the ads on other networks such as Google and Facebook.
Users download a decoration tool for their Facebook or other social networking pages and when they do that, it enables the tool developer to place ads on top of ads displayed on Google, Facebook, Yahoo and others. The WSJ said they are "inserting a layer of ads on websites or covering up other paying ads."
Here is a video from the WSJ on this:
AdSense publishers have a reason to be upset. They might be paying a CPM basis and get nothing out of this because their ads are covered by third party ads.
Forum discussion at WebmasterWorld.