Disagreed. They are de-facto mandatory to use. A former university I was at, for instance, published some required information on facebook. Saying "well, they should not do that" is as correct as it is useless and futile in practice.
I believe the information was visible to either all logged-in users or just all visitors, but that still requires facebook to serve the page to me.
You can say this about anything, though. "My university requires us to use Blackboard, therefore it's a de facto monopoly." "My work email account is through Gmail, therefore it's a de facto monopoly." They're still not. If another party requires you to use a service for some reason, that's between you and the other party.
I believe the information was visible to either all logged-in users or just all visitors, but that still requires facebook to serve the page to me.