We all have multiple avenues to get our message out. If you spray graffiti on the side of a privately-owned building to get your message out, your "speech" is not protected under any municipal, state or federal law. The owner of that building has a right to censor it by washing it away or covering it up. It is really quite simple. It's the same with a private social media company.
Government cannot impinge upon the rights of a private owner regarding speech, STAT. The only way around that would be the Eminent Domain, and that is really far and away from the purpose of "freedom of speech".