What void tho? The majority of people _I know_ below 40 don't use Facebook or any related service at all.
I always find it surprising when I hear of someone younger using facebook. I associate it with retired/elderly folks who love posting cryptic messages about their health or how their day has gone wanting attention.
I think you're forgetting that Meta owns Instagram and WhatsApp.
Most people under 40, 30 and even 20 I know are on Instagram, and many under 40 and 30 are on Facebook. Maybe it varies with country, education level or some other factors.
It's different by location. E. g. in Europe many people uses Facebook, many under 40s too.
In the US: 25–34 years: Around 25.7% to 25.9% (the largest single demographic group).
It's still used for stuff like events or groups
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The people you know aren't exactly representative of the whole country. Most people I know are on instagram.