If you spend too much time on a single Facebook page, you may start seeing some entirely new ads in the the right-hand side ad area. That's because Facebook has begun to rotate ads on certain pages based on how long a user lingers.
Facebook confirmed the new ad feature to ClickZ:
We recently made a change to some pages that show ads on Facebook that allows ads to be replaced with others after an extended period of time. This change was implemented a few weeks ago and we think this will help people see more relevant ads.
So, in theory this small shift could boost clicks, as Facebook has more than one chance to snag a user's attention. If they failed to click on any original ads on the right-hand side, it's possible that a new batch could have more success after they've spent a significant amount of time on the page reading a comment thread, for instance.
This new feature comes at the same time some users have witnessed Facebook expand the amount of ads shown on single pages. On pages with many comments, or more content in general, some users have reported seeing up to 10 ads on the right side of their screen. Facebook still says that the max number of ads that can be shown on a single page is six.[h/t AllFacebook]