Facebook stalks your friends and knows your social graph thanks to enough people leaking their contacts list (you're still exposed even if you don't share your own list).
It could very well be that one of your friends looked up the product being discussed and FB (correctly) guessed based on other data they had that you're the one most likely to be interested in it (or alternatively just shown this ad to everyone in your friends group, but only you picked up on it).
The first one was we were talking about getting pizza for dinner after my in laws showed up. 10 minutes later a Papa John's ad showed up. We don't eat pizza normally.
Second one was talking to one of my neighbors about their dog. For a few days I kept getting ads about dog leashes. I don't have a dog.
It could very well be that one of your friends looked up the product being discussed and FB (correctly) guessed based on other data they had that you're the one most likely to be interested in it (or alternatively just shown this ad to everyone in your friends group, but only you picked up on it).