Considering Facebook owns Instagram this sounds like great news for Facebook. Facebook the company owning the product teens are using instead of Facebook the product seems like a good long term strategy.
Yeah, not so great news for Facebook the company I'm afraid.
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).
Good point. To that, I'd add that Facebook needs "cool" users. Grandmas may use Facebook a ton and be very loyal, but they're not a cool, inspiring user group others want to follow and emulate.
