Some of my close friends and I have been using deltachat. It works off of email (has encryption) but looks like a chat program. No centralized server as it uses each individuals email server, they would have to take down all the mail servers . Works with gmail just fine, private domains are better but almost any email address will work. Nothing is totally private if it goes out on the wire. I don't know how email servers would handle this on a large scale but we've been using it on a small scale for a year and love it.