I don't know. In Facebooks case it might as well be hurting them with younger demographics. When everyones mom is there the younger people just might want to go someplace else.
You don't necessarily care that everyone is at some place, just the group you are interested in keeping up with.
Young people just make secret accounts and don't tell their parents. My younger brother is 15 and reports him and his peers are on both Snapchat and Instagram in addition to the usual suspects like Twitch, Tiktok, and Discord. The main reason I've heard for not using Facebook is largely that it's redundant when Instagram provides most of the same features in a slicker package. A lot of fb features like the event planning and facebook market aren't relevant to them yet.
You don't necessarily care that everyone is at some place, just the group you are interested in keeping up with.