Facebook announced earlier this summer, that they were going to be bringing encryption to their messaging app - Facebook Messenger. It was dubbed as "Secret Conversation", and has now debuted in the beta version of the Facebook Messenger app. The thing with Secret Conversations is it is completely encrypted. Meaning that no one, besides you and the person you're communicating with, will be able to unlock the conversation. Facebook has made it so that you can have some conversations encrypted but not all. There is a toggle in each conversation to make it a Secret Conversation.
If you want to start a Secret Conversation with a contact, just open the conversation with the like you would normally do. Then tap on the information icon, then tap Secret Conversation and you're all set. Now if you want these messages to disappear after a certain amount of time, you can tap the timer icon and set a timer for these messages to disappear. But it's not required. It's also worth noting that Secret Conversations are not enabled by default. So you'll have to enable it through the Facebook Messenger app. Additionally, if you have two threads with the same contact in Messenger, it's due to one being encrypted and the other one not being encrypted. Finally, Secret Conversations can only be seen on that one device. So if you use multiple smartphones or tablets and such, you'll only see that conversation on one device.
It appears that Facebook has rolled out this functionality for some beta users, but not all. There doesn't appear to be a new update to the app, that will enable Secret Conversations, leading us to believe that this is being done via a server-side update. So if you a beta user and aren't seeing "Secret Conversations" in the Settings of the Facebook Messenger app, just hold tight and you should see it appear relatively soon. It's good to see Facebook finally adding encryption to Facebook Messenger, especially since WhatsApp added it quite a while ago - and Facebook owns WhatsApp. Between WhatsApp and Facebook Messenger, the two biggest messenger services now have encryption for their users. Which is a pretty big deal.