The feature will mean users can choose to make private messages disappear after a certain period of time chosen by the user.
The end-to-end encryption of the messages means in theory no one, not even the Government or Facebook, can intercept the messages.
It is a feature designed to allow users to have more private conversations around sensitive issues safe in the knowledge the conversation is secret.
For now, the feature is being trialled on a “limited basis” but will become widely available to Facebook Messenger users later this summer.
Currently senders can choose only one device to use it on, as messages sent this way are stored on the device itself, where those flagged to "disappear" will be deleted from the device as well. Video and GIFs cannot be shared secretly at the moment.
The service will also have extra features for reporting abuse - and once this is introduced, there will be a delay in the deletion of messages to enable flagging.
Facebook’s secret service will use a protocol called Signal, created by Open Whisper Systems, which is already used in WhatsApp, Allo, and Signal’s standalone app.
Despite rolling out end-to-end encryption on its messaging app WhatsApp in April, the introduction of “secret conversations” marks the first step by Facebook to protecting a core part of its main product through encryption.
It is seemingly late to the game; Apple has used a form of end-to-end encryption in iMessage for years; Viber introduced the protection just weeks after WhatsApp, and Google announced in May that its new messaging app Allo would offer end-to-end encryption as an option.