8kun only exists in that form because of the arbitrary enforcement of "Community Guidelines" elsewhere. 8kun isn't an example of 'unregulated free speech' not working; it's an example of the externalities of trying to regulate free speech on social media.
Trying to 'fact check' your uncle who thinks the 2020 election was rigged forced him off that platform and onto another without those constraints. In that process, he's being exposed to even worse points of view.
I mentioned 8kun (and, implicitly, Q) because it's an English language example that should be familiar to the hackernews audience.
Outside of the English-speaking world, the mainstream social media platforms are far less moderated. So we have a place where we can test your theory, it's not hypothetical!
Result: Race-riots in Sri Lanka, anti-refugee violence in Germany, Rohingyan genocide in Myanmar, all fueled by disinformation on Facebook and organized on Facebook.
Uncontrolled social media creates a fact-free environment where the worst conspiracy theories and ethnic hatred spread like wildfire.
Don't attribute causation to what just could easily be the null effect. Genocide existed before facebook, and it will exist after facebook.
Saying "truth matters" is just an empty slogan without defining what truth means, and in itself is not a solution to what the real problem is which is the alienation people feel as a result of capitalism.
No, blaming capitalism for alienation is not an empty slogan. This critique comes directly from Marx who accurately described the phenomenon.
Capitalism is the predominant driving force behind alienation when every aspect of their lives becomes commoditizated. Facebook isn't the driving force behind genocides, in spite of what influence Facebook would like advertisers would like them to believe. (If you have a service that can sell genocide and allow people to 'steal elections', imagine what it can do for sugar water!)
Trying to 'fact check' your uncle who thinks the 2020 election was rigged forced him off that platform and onto another without those constraints. In that process, he's being exposed to even worse points of view.