How would FB know what my friends have purchased? I've experienced the "ad that's way too specific to a conversation held just moments ago to be mere coincidenc

How would FB know what my friends have purchased? I've experienced the "ad that's way too specific to a conversation held just moments ago to be mere coincidence" phenomenon several times. Each incident has involved the kind of product neither of us owned, none of our other friends would ever buy, and dull/boring enough that no one would have a reason to post/chat about it on Facebook.