Ask HN: How does Facebook know what I buy on Amazon or vice versa?

Following just happened: An hour before lunch I googled and visited websites that sell bicycles. I also visited Amazon during this research. I then bought a bike from one of the manufacturers' websites. A few hours later I browse facebook and see ads to this manufacturers' bikes in my newsfeed, via an Amazon sponsored ad.

I use one browser (Safari) for facebook exclusively, and browsed the bikes / Amazon / made the purchase on Chrome. I have different email addresses for facebook, amazon and, well, google.