Almost same thing happened with me today morning. I live in India and I was planning to buy a laptop cooler. So I opened homeshop18.com (a popular e-commerce site) and found the desired product there. I added it to the shopping cart. Facebook was open in another tab. Some time later I closed the homeshop18 website and continue to browse the Facebook. Later I found the first advert on the right hand side was exactly the same product on homeshop18 that I had earlier added to the cart.

I use different emails for Facebook and HomeShop18 website. So I guess this creepy thing has something to do with the Facebook's cookies. I think FB's cookies keep track of the other websites that we visit.

Anyway, I've decided to use "Self-Destructing cookies" addon on Firefox and remind myself to close facebook before opening any other website.