For what kinds of messages is snapchat the best medium? I've tried using it but can't understand its purpose.
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.