Even before then it's highly likely that they're gathering as much metadata as possible:-
* who's chatting with whom,
* via what means (text, audio, video),
* conversation time and duration,
* location of participants,
* how much data transferred etc.
There's a lot they can gather and imply from all that when WA users phone numbers will be known to FB, so they can graph connections via others' who run the FB app or previously shared their contact info. Also those who are perhaps logged in on, or simply visiting other sites they track via like/share buttons etc. Whilst what you say is encrypted, the circumstances around that conversation can be used, at least to advertise to you.
* who's chatting with whom,
* via what means (text, audio, video),
* conversation time and duration,
* location of participants,
* how much data transferred etc.
There's a lot they can gather and imply from all that when WA users phone numbers will be known to FB, so they can graph connections via others' who run the FB app or previously shared their contact info. Also those who are perhaps logged in on, or simply visiting other sites they track via like/share buttons etc. Whilst what you say is encrypted, the circumstances around that conversation can be used, at least to advertise to you.