Facebook has 2.5 billion active users.
Twitter has 340 million active users.
Regardless of network effect, I use neither of those services (for wildly different reasons).
Thus, if someone *really* wants to talk to me on social media -- and it's absolutely critical -- they will probably need a Fediverse account.
As far as I'm concerned, I don't care how many people use Facebook or Twitter.
What I care about is what I can use indefinitely.