Whatsapp is one of the most widespread mobile communication apps in the world. In some countries it is used by virtually anybody who has ever sent an sms.
It turns out now not even them can read our messages. Is it possible for the user to trust them? They offer a verification system, but I don't speak computer so I don't understand it.
More info: http://www.wired.com/2016/04/forget-apple-vs-fbi-whatsapp-just-switched-encryption-billion-people/
I doubt Facebook owned Whatsapp would make messages not readable even to them. They paid $19 billion for a free messaging app for a reason.