So now facebook has your data, has a shadow profile of you (already had that from your friends) and adds your whatssapp metadata to. You have to trust all current and future facebook employees who ever have access to that data. You have to trust facebook can now and will always in the future be able to secure that data. Then you have to trust anyone they sell that data to in all these dimensions. Moreover if data exists it can be retrieved by government through legal or other means so you have to trust all current and future public servants in countries in which you may not even have or want citizenship rights.
If you knowingly made that bargain and said, "Yes, I trust all these classes of people." Ok, fair enouhg. I imagine a number approaching 0% of facebook users, let alone whatsapp users actually thought through that bargain and knowingly agreed to it.
But yeah, you're mom might not see who you talk to on whatsapp via her facebook account. So there's that.
