Facebook may not be recording audio, but they do pay attention to your cell phone, GPS, and wifi, so they know that you're at a particular fast food place. Then

Facebook may not be recording audio, but they do pay attention to your cell phone, GPS, and wifi, so they know that you're at a particular fast food place. Then the brand-named shake is probably something they're trying to advertise, maybe even time-limited, so the fact that you're at that place is enough. Past that it's coincidence, the law of large numbers, and confirmation bias - you remember that instance and no others, and while a few people have their own anecdotes to share, HN's readership is large enough that FB could produce that population of anecdotes with a hit rate of "One time out of ten, advertise $restaurant and mention a single totally random item selected off their menu". The "People you may know" anecdote below is, similarly, some combination of location, wifi sniffing, luck, and confirmation bias.