LogFAQs > #979332441

There's a non-zero chance that you were randomly shown some Harry Potter content before this and subconsciously displayed enough interest in it that the algorithm saw fit to step up what it showed you (which may even have influenced your decision to watch the movies). Toss in the Bader-Meinhoff effect (or similar forms of semantic priming) making you more likely to notice HP content now that you've watched the movie, and the fact that you likely displayed more interest in the HP content when scrolling through Facebook now, and you've got a recipe to have your feed filled with it even if you didn't leave any significant opportunities to collect that data.

That, and your PS4 probably communicated that you were playing the movie. Even without directly connecting it to your Facebook, it could communicate that information by relating it to your IP address.