Facebook doesn‘t need the IDFA to rack advertising in their own app but they need it to: - track advertising shown in other apps that use their advertising tools - use their main app to tie the IDFA (which is all they have from third party apps) to your FB account

Privacy label requirements that some users may look at are one thing, but IDFA access specifically pops up a dialog that can be denied, like location access so it is way more endangered.