Uhhh, that already happened many years ago. Talk to anyone under 30, and Facebook is VERY out of style/out-of-date. They will most likely laugh at you or think you're being ironic if you were to bring up "hanging out on Facebook" or anything like that.
As of July 2019, 13 - 24 year olds make up 31.9% of Facebook's overall userbase. Another 32% make up the 25-34 age range.
Compare that to some of the other platform's 13-24 demographic:
Twitter 39%, Instagram 37.3%, Snap Chat 58.5%
While Snapchat has a slightly younger audience, they also only have 210 million active users, a 13% increase from previous year. Facebook on the other hand has 2.45 billion active users, a 9% increase from previous year.