People use both of them when they want to do something in privacy, instead of sharing it with the whole world. If you invent a message app that tells everyone

People use both of them when they want to do something in privacy, instead of sharing it with the whole world. If you invent a message app that tells everyone it posts every message to a publicly visible website, then I'd agree that people using it aren't expecting privacy. I don't see it catching on though.