There are developing countries where FB is so much a monopoly that huge percentages of people don't even know that anything else exists on the Internet. To them, FB is the Internet.
But anyway, that's an argument for encouraging competition and computer literacy, not for or against free speech.
It's actually reversed. People in developing countries are actually more savvy in many ways, as they entered into the Internet during an era where there was no Facebook or Facebook wasn't the dominate player. It's really only in the US where people are so illiterate that FB is a primary destination. But that doesn't make it a monopoly.
Not from what I've read. There are literally places in the developing world where FB fake news has caused the death of people. This example was from Asia. I don't remember the country, but it was news a few months ago.
OK, so, a monopoly on social media. But so what? You don't HAVE to use social media. It's not like having a monopoly on energy, banking or telecommunications, for example, where you can really put people over a barrel.
You don't HAVE to use energy, banking or telecommunications either. It's not like having a monopoly on food or water. The line you drew here is completely arbitary.
What makes you think the social media landscape of almost any country in the world isn't dominated by US-based services? They only need to provide a localized version to get into a market, which they then end up dominating through international network effects.
The only country I'm aware of where domestic services are dominant is China, and the statcounter data does reflect that. They are conspicuously missing WeChat and QQ, though (which are messaging applications and probably hard to track for a third party), so maybe there's some local service in Myanmar that similarly falls through the cracks.
But anyway, that's an argument for encouraging competition and computer literacy, not for or against free speech.