Legally (I believe) they can't delete an account, they can only hide it or deactivate it. There are rules around the data that go beyond Facebook. Think about it. Someone's told the cops I've been using threats using my Facebook account and I hear that and request the service be deleted. Poof it's all gone. Somehow I don't think that's allowed or does anyone think I'm not making a valid and likely assumption here?

Then sure they delete YOUR data, but all the inferencial data such as which sites you clicked like on, which sites you signed into with Facebook, the type of commentary or information you share you made/shared isn't yours and the knowledge gained also isn't the users it's Facebook's so that'll all be there. So sure they won't have your face or name, but they'll know everything else about you that makes you you.

Anyway most people don't even know that Facebook owns Instagram or WhatsApp, so no matter what they say unless they are tech savvy they have up at most an account on Facebook, but not it's subsidiaries (I believe).