When we do destructive testing in engineering, it's because we know for a fact that stuff fails for any or no reason, and while it might be rare, it happens all the time at scale. When the framers of the constitution designed strong separation of powers, they absolutely explicitly expected presidents to try to become kings, and so designed very strong safeguards against that happening.

The problem with your Facebook hypothetical is the absence of a mechanism by which the bad situation happens. How do we get to a point where Facebook to any practical degree has the power to prevent someone from dating at all? There has always been smaller and larger groups of misfits and outcasts, and they have generally been capable of connecting with each other beyond the majority hegemony (caveats here of course for repressive government power, but that's out of scope for this hypothetical, and belongs in constitution-related hypotheticals).

But let's then consider the hypothesis: Even if they do get to a significant position in dating, there are plenty of reason to believe they wouldn't use that power to prevent "anti-Facebookers" to date: privately owned publishers and printers happily print and sell the communist manifesto. You can sell your anti-Amazon book on Amazon, and publish it to Kindle. You can search for all the reasons Google is the worst company ever on Google. You can put "I hate Tinder" in your Tinder profile. And you can form a group about why Facebook is awful on Facebook, organise a "down with Facebook" rally using Facebook events and sell "Facebook sucks" stickers on Facebook marketplace. The logic of private markets very simply shows that it's plainly in a company's interest not to discriminate against people for the transgression of merely not liking the company.