This is definitely the correct take. I fall into the 18-24 demographic and I don't think I've used Facebook at all in the past 3-4 years. Neither does anyone else I know who is my age.
This is essentially a reverse network effect: because so few young people are on the platform, few other young people see a reason to use it.
Actually, my experience with FB (I have an account, but don't spend much time, there), is that it is packed with grayhairs, like me.
I think the younger folks have other venues they frequent.