In the same way that it's Facebook's prerogative to spread misinformation on their website, it's the user's prerogative to understand that they shouldn't trust what they read on Facebook. There is no way to stop the spread of misinformation on social media, attempting to curtail it to some degree is a PR business prerogative not an actual solution to the problem. Ultimately this is a cultural issue, we need to train the next generation to understand that anything read on social media should be implicitly distrusted; citing something you read on social media should be regarded as the intellectual equivalent of citing something you saw in a movie once. Yes, true things can be depicted in a movie, but nobody believes the movie is an acceptable source for itself, if a film claims to depict true events, it is expected that reliable sources will not contradict the film.