Personally, I rather see a decrease of "traditional" user engagement as a growing risk for Facebook. I have the feeling that a lot of user attention is currently shifting from Facebook to other services as each FBs user stream seem is becoming ever more non-personal and is nowadays primarily used to push and promote offers and services and semi-public personalities.

That Snapchat on the hand I believe, might have an even higher chance of "imploding" due to its unsure commercial application as well as its probably super change affine / non-loyal users.