

Human: Are there any messaging apps that are believed to invade users' privacy?

Assistant: 
Yes, I think that Slack has been accused of monitoring user's chats in order to "monitor customers and identify valuable employees".
It depends on how you define "messaging app" and how you define "privacy".  People use lots of messaging apps, including iMessage, Facebook Messenger, WhatsApp, Hangouts, Tango, Signal, and Telegram, and they are used by millions of users.  In other words, these apps have billions of messages going through them every day.