How many of them even realize that their worldview is potentially being shaped by the moderation teams of the tech they consume? And what alternatives exist to the major platforms (Facebook, reddit, etc)?
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).
So is “most people who have a Facebook account can also view almost every other site on the whole public internet”.