To different groups of people. That makes them less substitutable than wood and oil. It's possible to create oil from wood, it's possible to build a car that runs on wood, you cannot substitute on human for another.
> If everyone I know uses Discord to communicate then Facebook is as good as "wood" to Discord's "oil".
That applies to essentially no one. And if you mean that everyone you know is on Discord but not on Facebook: That's an argument for regulating Discord, not against regulating Facebook.
So does the scenario where the only viable way to communicate is to use Facebook. Send an e-mail or make a phone call. Facebook is a trivial and unimportant distraction at best and is not the primary form of communication for the vast majority of people.
No. If you cannot use another method of communication you cannot use another method of communication. Even if you dislike the obvious consequences: Facebook, like all other monopolies, cannot remain private property, much less unregulated.