A de facto monopoly means one that is, for all intents and purposes, a monopoly - despite the existence of alternatives. Is anyone going to read your content on any of the services you mentioned?

Plenty of people use Parler, Facebook, Mastodon and Gab. Seems quite American to me to be supporting a marketplace of alternatives.

Facebook is in a different category. Are you really arguing that Parler, Mastodon, and Gab have userbases that are on par with Twitter? How exactly are you defining “plenty”?

There are alternatives with appreciable communities and free and open access. Use them if you don't like Twitter's policies, and don't for a second think that freedom of speech entitles you access to a private platform.

Freedom of Speech has never been a guarantee of platform access or forced association.