Nobody's business!
- Who all do I talk to?
- When do I talk to them?
- How long do I talk to them?
There are certainly a lot of problems with WhatsApp, but these are the ones that concern me the most.
To keep it simple, I wouldn't discuss the lack of transparency (WhatsApp being closed-source software) and concerns over actual implementation of E2E encryption.
Regardless, I believe you'd agree that it shouldn't be anyone's business in knowing all this about you, especially, Facebook! A company mostly known for utter disregard of its users' privacy.
For those who always bring up the shitty rhetoric 'If you've got nothing to hide, you've got nothing to fear!', let me respond back with a literally 'shitty' example:
Everybody shits. It is a completely harmless activity. Nothing wrong, illegal or unlawful in doing it. We all do it & also know that everyone else does it.
Still, you close the door while shitting, don't you?
So please do not conflate privacy with secrecy!
P.S.: Signal (FLOSS app developed by a not-for-profit Foundation) with its state-of-the-art End-to-end encryption protocol has been my preferred chat app since its early years. And from a long time, I've been meaning to completely move away from WhatsApp. Their latest ultimatum to either accept sharing data with Facebook or stop using WhatsApp gave me the much-needed inertia.