Earlier quoted context omitted.
I believe FB messenger now has secret/disappearing messages as a response to snapchat, but before that it was a pretty clear distinction: if I send you a goofy selfie on snapchat it's gone in 10 seconds. If I send you a goofy selfie on messenger, you could always scroll up to see it again. And share it with others if you so choose.
I guess the part I'm missing is why you care whether the goofy selfie disappears.
Also it's private. Facebook is mainly used by people with friends and family. Celebrities/notable figures rarely interact with others outside of Twitter. But even with Twitter, the interactions are permanent -- even when tweets are deleted.
On and it's great for sexting, which let's not kid ourselves, was the original appeal and still is for a lot of people in their target demographic.