If my local government or some local business was
limited to using one specific newspaper, this would indeed be a problem.
And in reverse, having a concentration of readers (users) on one platform, means that whoever controls that newspaper (platform) gets to be kingmaker for the government and the businesses.
Free speech as a constitutional requirement binding on a government to allow it is great; that government then using one specific private corporation that can use its own status as a non-government actor to have free speech rights rather than free speech responsibilities, means it is a de facto loss of free speech for everyone else.