I don’t think it’s that either. I think it’s about Facebook’s algorithm massively amplifying certain voices.
Facebook will happily amplify any voice, even incoherent rambling, and find customers for it. I welcome you to try - the only thing you have to do is write hundreds of comments in public groups. You will be successful immediately. The problem is the 'rational' side considers that 'below their level', often any Facebook usage at all is considered waste.
So that they can be heard denying vaccine effectiveness, though. Again, it's a practical issue. Some messages, including this one, shouldn't get Facebook-level assistance with reaching listeners.
They generally don't deny vaccine effectiveness. They take aim at vaccine safety. If you don't even know what these people are upset about how can you be so sure about denying them a voice?