Not so sure about Facebook analyzing your WhatsApp traffic, because they make such a big thing in their advertising about the secure end-to-end encryption and privacy. Surely they would get roasted if it was found not to be true. Of course it's always possible with a closed source app.
The technique discussed in Darknet Diaries episode 146 (about the ANOM phone) involved duplicating the message to [an] archive endpoint. It mentions Google providing text archival as a service for company phones, and it seems this also happens for WhatsApp. However, are Facebook also purloining your private WhatsApps? Are they using them to target you? Who can say?
https://www.waterstones.com/book/chaos-monkeys/antonio-garci...
https://darknetdiaries.com/transcript/146/
https://www.telemessage.com/mobile-archiver/whatsapp-archive...