Evidence has surfaced to suggest Facebook is working on a way for Messenger and WhatsApp users to message each other. So messages from WhatsApp could appear in your Messenger app. 

Facebook has mentioned in the past that it is seeking to bring more interoperability to its suite of apps, and one way could be for this cross-platform messaging between WhatsApp and Messenger. 

Hidden information - revealed on WABetaInfo - shows a list of references to WhatsApp within Messenger. It includes knowning (locally) your chat properties, blocked contacts, media items, messages, contacts and group members (among others). 

Facebook Messenger and WhatsApp might be able to send messages to each other in future photo 2
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Crucially, with WhatsApp being an end-to-end encrypted messaging service, it's said none of the message content is shared directly with Facebook. All the information is seemingly being stored locally on the device. 

To communicate with other users who use the app, it's being suggested that it'll mean Messenger would need to work on a similar basis to WhatsApp in order to encrypt and decrypt sent and received messages. 

We don't know if, or when, the interoperability will be available to the public. There's a suggestion that it may never become a reality. However, with Facebook wanting to bring more cohesion between its various products, it would certainly be useful. Particularly if you have friends who are only on one of the services, and you want an easy way to message them.