Facebook is an effective monopoly in its space.
It's only a monopoly for people who want to play on Facebook. If you want to run an ad, there are numerous other outlets. If you want to connect to old school chums, there are other outlets. If you want to post shit and have people comment on it, there are numerous other outlets.
YouTube is an effective monopoly for people who want to make a living posting video,
I know a few people who make a living producing video, and it does not involve YouTube. I know what you mean - there is an ecosystem of people who now depend on YouTube for their income... but that isn't a monopoly. The skills that go into the production of a YouTube video are transferable elsewhere. For viewers, there is plenty of boob tube content, even if YouTube closed tomorrow.
as eventually happened with Microsoft in the browser space
No question - but that was actually a monopoly. Except for a tiny niche, you couldn't really sell a PC in the US without paying MS. They owned the whole market. A market that was once rich with competition from names like Apple, IBM, Atari, Commodore, etc. was whittled down to a single player.
I see no problem with forcing the creation of a platform for everyone, if the market isn't working to do so.
I want to get this right. You think that, if anyone - ANYONE - out there, no matter how harebrained the idea, no matter how incomprehensible their drivel - can't find a publisher for their content, that requires a government intervention to force people to host it? I remain unconvinced.
There are government-owned public squares in the physical world, but not online.
There are things that are almost perfectly analogous to yelling at people in a park, though. Usenet, for example.
We could make some, but it seems like there are better options.
Like forcing people to host other people's content? I remain unconvinced.