Earlier quoted context omitted.
They have captured an important segment of the market (12-2x years olds). I also think that people In this age group find it a ton more engaging than the alternatives.
FB really made it when the captured the old people. The issue is the the 12-20 years are quite mercurial. 15 years ago it was AIM,MSN, Yahoo chat, then myspace, then facebook. Now its snap chat and instagram. Its hard to keep the market since when you get friend request from Mom or Dad do you really want to be in that social network anymore?
I don't think we are mercurial but that we have less experience and therefore are much more willing to try new things. Snapchat is way better as an outlet to post more regular things you are doing to your friends because it is ephemeral.
Why? Because we all look back and see all the stupid stuff we posted when we were younger. The only reason I haven't deleted my facebook to delete all of that old stuff is because Facebook's other offerings are pretty good. While I hardly know anyone that posts regularly to facebook anymore, almost everyone uses facebook messenger and groups to communicate in isolated groups.
Snapchat has been blowing up because it fills that niche of "I want to post more updates about my life without really having to worry about how those updates will be interpreted in the future".