I hear what you're saying, but Facebook (and good-forbid, Twitter) are profit-making enterprises, governed by all sorts of rules. Free speech has never meant that you can do or say whatever you want, wherever you want. It means you can speak or write your thoughts without going to jail. That's all it has ever meant.
EDIT: To be even more clear,
-you can't hold-forth in a synagogue about how evil Jews are, and wouldn't expect to.
-You can't do it in your office, either, and expect to keep your job.
-You can't use the free speech argument to get responsible news organizations to publish your demonstrably false beliefs.
If Facebook is really such an important cultural resource that free speech matters there, they should ensure they're actually doing good on-balance in important matters.
Zuckerberg has championed AI as a more fair way of performing this censoring, however it is easy for AI to pick up biases (Microsoft's Tay bot for example) depending upon the training data. AI also provides scapegoats and plausible deniability: "we're constantly working to improve our algorithms" "the algorithms aren't perfect". Since news feeds are personalized & private, it is difficult for anyone who isn't facebook to audit or even detect biases.
It's a tough problem, but I believe 'fake news' is best fought with truth & facts, not censorship. Censorship has a long history of abuse.
We generally agree that speech should be free, but obviously we put limits on it. For instance we do not allow yelling “fire” in a crowded theater. The reason is that people are directly harmed by that speech, and it has no upside.
It’s perfectly reasonable to consider much of what goes on, on Facebook in the same vein. If holocaust denial, or other types of hate speech or misinformation are spread rapidly, and cause moral panic, possibly harm to others, should we not consider limiting it?
It’s not a black and white issue of “speech should be free at all times”.
Facebook and social media is fundamentally changing (has changed) how humanity communicates. Thinking though and questioning the ethics and moral obligations of those services is quite important.