2. Facebook's reputation is so bad when it comes to privacy at this point that trusting them is just naive.
3. Even in the best case, your metadata is certainly used. Facebook didn't pay over $20 billion for a service with no monetisation model purely out of the goodness of their hearts.
4. Whatsapp has good network effects at least here in the UK - when your flatmates or coworkers have a group chat, you can't just say "Well, let's get everyone over to Signal.". You either use Whatsapp or go without the group chat.