According to a document Facebook has sent to some of its biggest advertisers, a copy of which was obtained by TechCrunch, the social network may start injecting ads into people's conversations on Messenger in the second quarter of 2016. Specifically, Facebook currently plans to allow businesses to send ads as messages to users who previously interacted with that company using the Messenger service.
The leaked document advises that businesses “get consumers to start message threads with them now so they’ll be able to send them ads when the feature launches,” as per TechCrunch.