I don't know if those who use the argument "no one should be stupid enough to believe anything they read on Facebook" say this because they don't understand basic human nature or if they're just being ingenious.
The reason Facebook is such a successful medium to spread false information is because you're not getting that false information from someone anonymous, you're getting it from your personally selected "friends". It is a natural reaction for anyone to trust information if it's given to you by someone you know. The method these organisations, like Cambridge Analytica, get the false information onto your feed is by using the extremely detailed lifestyle preferences of people to braid in the lies. Through seemingly innocent pages like cute dogs, car fans, sports, <insert your favourite pages here>. You then share that to your friends and viola, the ball is rolling to the friends of the friends and so on.
Facebook doing this is nowhere near enough. Facebook and other social media like YouTube, Twitter, Whatsapp needs to be regulated like any other media. Companies owning the services should be held personally responsible for what they allow onto their services. In the US that would be really easy to implement as corporations are already legally considered human beings with human being rights. Responsibilities comes with rights.