But free speech as an ideal is made to protect people from the unique threat posed by state censorship. This may have applications to platforms with influence as widespread as Google or Facebook, but it certainly doesn't apply to most other private entities. Absent such an overwhelming imbalance in power, what most people consider "censorship" is more often than not just counter-speech and holding people accountable for their words and actions.
When the constitution was drafted, there were no companies like facebook or google. The state was the largest and most powerful entity. The first amendment is a product of its times, if it was drafted today it would be different to reflect the massive power that corporations hold over people.