The same teenagers also don't have much spending power now. And besides, teen trends change pretty quickly, so their main targets are teens it does not bode well.
That is the key transition. Once they start greying, users will perceive any new platform as unwelcome change. But until then, you have to eventually give them better reasons for staying than just "this is younger and wilder than that other thing".
Facebook's ticket for growing up in lockstep with their core user generation was coordination and communication for loosely coupled groups with some real life connection. People who move into a new development, the "parent cohort" around a daycare class, new hires who joined a company at roughly the same time and so on. All that is young adult stuff that fits well with what Facebook offers.
Nonpermanent asynchronous video messaging? If interpersonal exchange is always a blend between information propagation and entertainment, Snapchat seems dangerously lopsided towards entertainment. That is good for getting people excited (quick growth), but probably not so good for long term retention. MySpace was similar.