Everything you said other than ephemerality applies to every other messaging platform, including Facebook Messenger. How does ephemerality help you shoot the shit better than a text message?
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.