Just 51% of US individuals aged 13 to 17 say they use Facebook – a dramatic plunge from the 71% who said they used the social network in Pew’s previous study in 2015, when it was the dominant online platform.
In this year’s study reported Facebook use was, according to Pew, “notably lower” than the percentage of teens who said they used YouTube (85%), Instagram (72%) or Snapchat (69%). In the previous study, just 52% of teens said they used Instagram, while 41% said they used Snapchat.
https://www.theguardian.com/technology/2018/jun/01/facebook-...
Being a social media user is age-independent, whereas one's teens is something they grow out of.
So, not only existing teens grow out of being teens (and eventually Snapchat users), but new teens won't necessarily grow into it to replace them.
Unlike older people, who value utility and what they already know more, new teens are just entering the social media field (and thus are more open to whatever might come next), and they are all about what's cool (so when a service loses it's cool it abandoned MySpace-like fast).