Facebook Messenger is gradually introducing end-to-end encryption in the form of Secret Conversations to its roughly 900 million active monthly users. This new security feature relies on a protocol called Signal (already used in the standalone Signal app, Facebook's WhatsApp, and Google's Allo). If you're one of the lucky ones who's received the Secret Conversations update, then your Messenger chats will be private.
You'll know if you have it, thanks to an alert that sits atop Facebook Messenger's home screen. But it's important to know that Secret Conversations is not a default feature: You must manually enable it each time you use Messenger.
To start a Secret Conversation, tap the compose button on the top right of the screen (iPhone) or the plus sign on the bottom right (Android). Tap the Secret Conversation button, then select which friend to contact (no group chats allowed) and enter your message.
In the bottom message field, you'll see buttons for sending text, photos, and emoji. Apparently, you can't yet send GIFs, video, or payments secretly.
If you'd like to send a Snapchat-style disappearing message, tap the clock symbol in the message field and select a time from 5 seconds to one day to expire your message. Then tap Send.
Your contact must already have the Secret Conversations option for your message to go through. Otherwise you'll receive an error message stating that your recipient must update their app.