Maybe some are too young to remember the great migrations from/to MySpace, MSN, ICQ, AIM, Skype, local alternatives like StudiVZ, ..., where people used to keep in contact with friends. Facebook was just the latest and largest platform where people kept in touch and expressed themselves in some way. People adding each other on Facebook before others to keep in touch hasn't been a thing for 5 years. It's Instagram, Discord, and WhatsApp nowadays depending on your social circle (two of which Meta wisely bought because they saw the writing on the wall).
If I open Facebook nowadays, then out of ~130 people I used to keep in touch with through that platform, pretty much nobody is still doing anything on there. The only sign of life will be some people showing as online because they've the facebook app installed to use direct messaging.
No, people easily migrate between these platforms. All it takes is put your new handles (discord ID/phone number/etc) as a sticky so people know where to find you. And especially children will always use whichever platform their parents don't.
Small caveat: This is a German perspective. I don't doubt there's some countries where Facebook is still doing well.