The first and main thing to understand, is that the first amendment is written in a spesfic way. It guarantees that the government will not censor or stop your right to free speech. Having said that, it does not prevent any social media platform from censoring you. In Fact when you sign up to any social media site, you agree to their terms and conditions weather you read them or not. Because of that, when you break them, they can censor you or ban your account.