Two points here. One is that congress is certainly coercing media to censor speech under threat of potential new laws. That falls under this amendment.
The other point is that, even when these so-called private entities choose to "refrain from publishing material" for themselves, they are a threat to democracy by nature of their monopolistic control over communications and their conspiratorial relationship with certain political parties. Would you be happy if you were denied the use of the telephone or Wifi because of your opinions?
People who disingenuously argue that Twitter, et al are not violating free speech but simply acting as private companies are purposely missing the bigger picture.
Whether Twitter, Facebook and Google violate the right to free speech as written in the constitution is not really the issue. The issue is they are squelching free speech in a manner that threatens the existence of democracy. It is indefensible.