Holding Facebook accountable for the fake news spread on Facebook is entirely reasonable if there is proof that fake news is measurably harmful. Just like holding Google accountable for fake news that is turned up at the top of Google search results.
It’s absurd for a product or content creator to not take ownership for the externalities created by their work. Hate speech that radicalizes people to commit acts of violence is a huge problem in both the US and Europe. How many people will have to die because Twitter, Facebook, Google etc stop placing the burden of dealing with violence that’s triggered by fake news spread on their platforms? Stabbings in Oregon, bombings in Paris/Manchester, shootings in Kansas City not enough? Will it take someone close to a google/facebook/twitter founder or employee getting attacked? Why?