In the age group 12-34, Facebook dropped from 79% usage in 2017 to 62% in 2019 while Instagram increased from 64% to 66% over the same period, so at least some former Facebook users didn't just migrate to Instagram. They don't have data for WhatsApp before 2019 (23%), so it's still possible that the total number of users in that age bracket actually increased when you pool all of Facebook's properties.
In the age group 12-34, Facebook dropped from 79% usage in 2017 to 62% in 2019 while Instagram increased from 64% to 66% over the same period, so at least some former Facebook users didn't just migrate to Instagram. They don't have data for WhatsApp before 2019 (23%), so it's still possible that the total number of users in that age bracket actually increased when you pool all of Facebook's properties.