As is the case with WhatsApp, time limits will be involved, meaning you won't be able to remove any of the things you said in past chats.
Currently, while Facebook users can delete messages that they sent from their own inbox, the recipient of the message can still see them. In my experience, you know pretty quickly whether you've sent a message to the wrong person (or the wrong message to the right person) most of the time.
Facebook didn't announce the feature yet, but The Verge spotted it in the "coming soon" section of the release notes from Messenger for iPhone version 191.0. At least, that's how its worked for users whose name isn't Mark Zuckerberg.
Yet users will need to be quick as Facebook are giving those wanting to retrieve their messages just 10 minutes to do so.
Facebook will soon roll out a feature on its Messenger service users have always been calling for. Facebook's unsend for both its messaging apps will actually deliver the message when you hit send. We reported back in October about the functionality, pointing out the benefits and disadvantages of the Unsend messages option.