Unfortunately, the problem is humans.
Realistically, what can facebook (or any other internet/social media/news platform) do? The most they can do is promote “reliable” content and deprioritize “unreliable” content, but then they run into issues of how to rate the reliability of sources/content.
And even assuming they solve that (maybe by asking govt+independent orgs to certify reliability?), what are they supposed to do if people keep sharing unreliable content regardless?
Total worldwide online connectivity unfortunately means the internet brings together even believers of supposedly fringe ideas - flat earthers, anti-vaxxers, conspiracy theorists, white supremacists, fanatics, etc and if you censor their content they will complain about freedom of speech and that starts us down a slippery slope…
The long term solution would be to educate people, but that relies on more people being willing to be good actors on social networks and speak up against fake news and other unreliable stuff. It isn’t in the hands of the tech companies