Could well be advert blocking software in your browser or Security Software.
I use Kaspersky Internet Security which includes an Adblocking element to it. I haven't checked recently, but I think that Fring advert is missing for me too (which is to be expected due to Kaspersky).
You might be able to create exceptions for pages that are allowed to show adverts, exceptions for certain ad providers etc.... or even turn off Ad blocking on a temporary or permanent basis, (depending on preference and settings).
Finding what software is blocking the ads could be fun though, if you are not aware what is doing it. I think some Browser Toolbars do it. As do specific browser plug-ins and some security packages.
Hope this helps.
Rich Mark, it might be worth checking if this was the issue with others that had this problem, as it might not be a problem at your end at all. Of course, it might still be an anomily at your end too!
Mike
(Edited to change Mark to Rich, as I was clearly miles away in my mind when I posted this!)