Then what is freedom of speech if every plattform deletes your content? Does it even exist? Facebook and co. are so ubiquitous, we shouldn't just apply normal laws to them. They are bigger than governments.
Freedom of speech means that the government can't punish you for your speech. It has absolutely nothing to do with your speech being widely shared, listened to, or even acknowledged. No one has the right to an audience.
This has always been the case. If the monks didn't want to copy your work, it didn't get copied by the monks. If the owners of a printing press didn't want to print your work, you didn't get to use the printing press. If Random House didn't want to publish your manifesto, you do not get to compel them to publish your manifesto.
The first amendment is multiple freedoms. Your freedom of speech is that the government shouldn't stop you from using your own property to do something. You are free to print out leaflets and distribute them from your porch. If nobody wants to read your pamphlets that's too damn bad, welcome to the free market of ideas buddy.
The first amendment also protects Meta's right of free association. Forcing private companies to platform any content submitted to them would outright trample their right. Meta has a right to not publish your work so that they can say "we do not agree with this work and will not use our resources to expand it's reach".
We have, in certain cases, developed system that treats certain infrastructure as a regulated pipe that is compelled to carry everything, like with classic telephone infrastructure. The reason for that, is it doesn't make much sense to require every company to put up their own physical wires, it's dumb and wasteful. Social networks have zero natural monopoly and should not be treated as common carriers.