http://www.mediapost.com/publications/?fa=Articles.showArticle&art_aid=153247&nid=128337

What’s interesting is that Facebook advertisers are not necessarily sticking around… the article alludes to a “test and learn” phase. What I’m seeing among certain Facebook advertisers is ad saturation and ad burn-in… so an initial campaign does well, but over time (even with new creative), the campaign returns begin to decline. Facebook will need to improve its advertising products or offer better fraud detection to retain advertisers over the long run.