This is a very different scenario than a single company owning every broadcast tower in a town, pre-internet.
The ability to access Facebook strongly implies general internet access, which is a compelling argument against Facebook exerting any sort of authority over general truth.
So is “most people who have a Facebook account can also view almost every other site on the whole public internet”.
They're effectively a captive audience.
What specifically makes them captive? Why do you believe that? Are the audiences of other extremely popular websites also captive? Why or why not?
Perhaps you meant this hyperbolically but I do not believe it to be an exaggeration.
>What specifically makes them captive
They are captive because of a combination of their ignorance and the network effect. If all of your friends are on Facebook for example you'll have to leave them behind if you delete your account.
In fact, if your argument is "most people are ignorant" it's an argument in favor of not letting their primary source of news be dominated by wealth and network effects instead of grounding in at least some objective reality (imperfect though it may be).
People who want "the real truth" have few impediments to seeking it out on the modern Internet. People who don't want to do their own fact-checking are better served by somebody doing it (and then we can keep a close eye on the watchmen).
Facebook has literally billions of monthly active users [0], close to two billion daily active users [1] and very likely a number of total accounts that's 10+ billion [2]. Instagram has another billion mau, and half a billion dau [3].
I struggle to think of any other private entity with that much global reach into so many peoples lives, the only other example that comes to mind is Google, the two of them combined have direct influence over 70%+ of internet traffic [4].
Staying clear of their impact is active work because even if you don't use their services, chances are many and often even most people around you still do.
[0] https://www.statista.com/statistics/264810/number-of-monthly...
[1] https://www.statista.com/statistics/346167/facebook-global-d...
[2] https://youtu.be/nmXACRrzLMA
[3] https://www.omnicoreagency.com/instagram-statistics/
[4] https://staltz.com/the-web-began-dying-in-2014-heres-how.htm...
Their popularity does not give them any ability to censor information widely, unless perhaps they collude with search engines to blacklist certain terms across all popular sites.
The fact that many people use Facebook is an indicator that those same huge groups also have access to every other website, which reduces the persuasiveness of the argument that Facebook gets to be the arbiter of truth, so long as all of those Facebook users have internet access that can access sites that are not Facebook.
If anything, the real arbiters would be the ISPs, who get to decide which sites you can even access, in real-time.
Three of those are entirely Chinese and can be discounted from this conversation due to government. That leaves two sites run by the same company, all of which are headquartered in the same general area and have similar culture, and both have ongoing censorship/misinformation controversies.
I can't read Chinese. So discount those sites.