It is doing the EXACT same thing for me and I am definitely logged in. It is getting really flipping irritating. Whenever there is another ad (not the purina one) it is normal, no ad in the middle of the page. I bet purina did some shifty coding to get it to appear there to get more hits.
Nifty just started supporting it and he says that you'll see some new banner ads. Either that's just the programming for the ad or something's gone wrong... I certainly don't see it.
If you are computer saavy at all there is a program that you can called NoScript that is a pop up/software blocker. It works automatically by stopping Java (a type of internet ad language so to speak) apps from opening on web pages. It controls all the little extra things that pop up on web pages and stops them from opening automatically until you give the ok for it. It can also stop menacing software/adware from automatically starting every time you open a site. Ads will not show until you allow them and then every time you open the site things you allow come up while all the other ads and things do not. Once you get the hang of the software it is very easy to use and works without any effort on your part, just runs in the background and auto updates as well.