Messages can be either public or private, depending on whether they are intended to be visible to everyone, or only a few people.
At the moment, MeowCat only deals with public messages.
Later it will include features for handling private messages as well.
It's important that private messages stay private. To ensure this:
If you send a private message on Facebook or via gmail, there's a copy of it on Facebook's or Google's servers, and governments can ask for it.
Governments can ask MeowCat for private messages but they won't get them, because MeowCat simply will not have that data.