There is no guarantee of freedom of speech in the sense you are using. The First Amendment in a nutshell simple guarantees the right of an individual to speak out against the government (Freedom of Speech), without fear of reprisal by the government. This is the basic foundation of it; it is this along with the right to peaceful assembly that gives the right to protest. The courts have further interpreted that this right extends to prohibit the government from censorship of the people in anyway. However this right, despite the fact that many skew its interpretation, does not guarantee anyone the right to speak their mind at will without repercussion. A perfect example is what is currently going on in the NFL; strangely enough the original protest might have had some protection under the First Amendment, as it was a protest against police brutality and therefore might be loosely be interpreted as a protest against government action. However the current protest adopted by many players, has been stated as a social protest against systematic racism throughout mainstream America, a very broad and arguably non-government entity. That combined with the fact that the government has not interfered in anyway with their protest, leaves it well outside the domain of protected speech. Fans are free to boycott the games and merchandise as well as the sponsor’s product, the managers and owners are free to state compliance as a rule of their business. None of this is a violation of the First Amendment.
As for any form of Social Media the same rules apply. However if the government is found to be trying to influence the business in any way shape or form in its decisions, then it becomes a violation of the First Amendment. Facebook Twitter, etc. as a business can accept or ban any individual from use of their service for any reason, they can even actively censor things they don’t like, while you have a right to your opinion no company is obligated to give you the platform to state it.
Now there may be other laws that come into play here, that could have some effect, but from the basis of the First Amendment, this is how it is supposed to work.