The 1st Amendment only limits what Congress can do, it doesn’t say anything about what public or private companies have to do with respect to freedom of speech: “Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.”

IANAL, but I guess if Congress makes a law forcing Twitter or Facebook to abridge US citizens’ freedom of speech, then Congress would be violating the 1st amendment. Is that what you meant?