No tricks, but I disagree with you here.
A government-run, publicly owned online communication platform has several problems that prevent it from being the utopia you expect.
1. What value does it actually provide that Twitter and Reddit and Tumblr (and even Facebook) don't provide? I've learned more about marginalized communities both locally and globally on Twitter than I have anywhere else. Seems that they have a pretty decent platform today.
2. It's existence doesn't mean those platforms go away, so it by itself doesn't stop Facebook from doing the ridiculous things it's doing today. Folks may choose to patronize those private platforms over using the thing you describe.
3. Economically disadvantaged folks won't have access to it without a separate set of programs to ensure every household is connected to the Internet at a reasonable speed, and that they have the tech required to access it. (COVID-19 has laid bare the economic disparity even in my relatively affluent school district.)
4. Are we talking "run by the US"? If so: globally, marginalized people may not be able to communicate there due to policies from our government (which we can over time theoretically control) or their governments (which we cannot.) If not: who runs it then?
5. I think you'll find that the US government routinely denies people speech. Police violently responding to peaceful protests around the country is just one example. The US would definitely have to control for speech that violates US law, such as SESTA/FOSTA.
6. This is all ignoring the fact that people are people, and even the best examples of online communities have examples of folks being rejected by the dominant voices in the community irrespective of the rules.
I think a public-owned online space is fascinating, but I don't think it'd be a magic bullet to solving these problems.