Facebook has expanded to new countries in the last decade which has brought these millions of users, but younger generations in countries that have had Facebook for a long time are not using it anymore.
In the US most Facebook users are between 25-34. There are barely any teenagers.
https://www.statista.com/statistics/187041/us-user-age-distr...
This trend is a bit less pronounced in global demographics, but I think it's clear that younger generations don't like Facebook.
https://www.statista.com/statistics/376128/facebook-global-u...