Facebook may be a private company but it doesn't absolve them from criticism due to the size of their userbase and influence on society. Antitrust law may also apply since they control Instagram and Whatsapp as well.
Sure, you can post on your website (that nobody would discover), or Gab (which is blocked in Apple and Android stores), or 8chan (which is down)...but for better or worse people still use Facebook so people still complain about it.