A monopoly is composed of “one” entity. It’s in the name. This is not a fair characterization of what happened.
The right term in this case is an oligopoly.
Semantically it makes little difference though.
People want free speech. You get free speech so long as your speech doesn’t unduly impinge on other’s liberties. That is the trade off we accept when we exchange the rule of force for the rule of law in a self governing, democratic society.
People are not complaining about business practices. People are complaining about not being able to say whatever they want over whatever medium they want even if other people get dead.
Unless you're quite tech savvy, you can't sign up for Parler anymore by using a competitor. So, while I mostly disagree with Greenwald, I don't think this characterisation is unfair.