Ask HN: How is this 'cross advert' possible?

I was logged into FB on one tab and then searched for a bike on Amazon, on a new tab. I think that I was not logged into Amazon. A couple of hours later I logged back into FB and there was this ad: http://www.grupster.com/crossadvert/fb_ss_blrd.png.

How do they make this work? Cookies? Do I have adware on my laptop/browser? Do FB and Amazon share traffic data? All of the above?

Thanks, this explains it! Annoying and scary....