What they want to control is the algorithm that decides who sees what you post.
Everyone thinks that they have a platform, but that's just an illusion, what everyone else sees is decided by the algorithm. It is essentially like a TV station that is personalized to each viewer.
You can manipulate public opinion. This is especially worse with generative AI, as now you can generate content without needing users that would spread message you want to amplify. You just generate the content you want.
This is why billionaires want to control it, because it gives them power to persuade you what to buy, how to vote, what causes you want to support etc.
You as an ordinary person don't matter, when they control the algorithm they can simply mute you.
That's why activism on social media is just an illusion. If you go on the street it is much harder to filter you.