Read this article from AdWeek.

“If you look at the top-ranked sites online — Facebook, YouTube, Wikipedia, Blogger, MySpace — a surprising number have not been cracked by the creative industry. Sure, there’s media space on these sites and there has been the occasional one-off idea that’s been a hit with users or the industry press, but there hasn’t been much in the way of a repeatable, successful formula.

Display ads don’t work that great, Facebook apps are a media ghetto, paid YouTube ads have to compete with user-generated content that lawyers do not look at before the video is uploaded. It’s rough!”