While I don't know much about how secure the messages are in WhatsApp, it's easy to imagine other ways Facebook could harvest data from users. They could track what you tap on in the screen, or how much you scroll, or what buttons you click on. They could still use that information to serve ads effectively in the future.
Like, how many people click on forwarded messages or pictures.
It stands to reason that for every WhatsApp conversation they'd have access to:
- who is communicating with whom,
- dates, times, and durations,
- method (text / voice / video),
- amount of data transferred,
- type of attachment if applicable, and
- location of each device,
along with unique device identifier, and perhaps other information.
See the Privacy International report[0] or video[1] on how much data FB glean from on other apps that merely use the Facebook SDK, each time an app that uses it it opened for a clue... how much more will they want from a service they paid billions for?
Like, how many people click on forwarded messages or pictures.