I've only had a FB account for a very short while and that was 8 years ago or so, their disrespect for user privacy creeped me out back then already, never went back.
If WhatsApp had forced me to link to a FB account (and thus make a new one), I'd have told my friends "yeah that's nice you can message me on a different platform from now on". And I wouldn't have been the only one.
There's no social network for people refusing to use FB, so we don't know each other :) But there is way more of us than you'd expect (and from many different corners of society too, not just the tech crowd, friend of mine is a doctor and she's very intent on privacy, even if she sometimes lacks the tech knowledge to make the "perfect" choices, she's much more hard core about it than I am, as if I didn't have enough reason already for respecting the hell out of her).
If it really is just that asshole prompt that made the difference, then man, did Facebook fuck up. They should have just blocked people from using WhatsApp until they volunteer their data.