That's not freedom of speech. Freedom of speech is a general principle that covers more than government censorship. What you're thinking of is the first amendment, which covers government censorship (including by its agents). 1A neither invented nor set the absolute boundary of the principle of freedom of speech.

In that case freedom of speech is generally defined according to Mills in On Liberty which still doesn’t acquiesce to the application suggested of social media. They as a large business of many users should not be provided a greater speech mandate than a dissenting individual should speech be actually free.