Contents of Images Folder in AddressBook - 3 million-plus files and counting, tens of gigabytes

It's possible as you mentioned these files are coming from macOS trying to synchronise your Facebook contacts with your Contacts.

This setting is in System Preferences > Internet Accounts.

As you can see there is an option to include you Facebook contacts in your address book, and additionally the Update Contacts button which tries to match your contacts and update details about them like birthdays and photos.

If this is going wrong for some reason it could create a large amount of files which as you've pointed out while not individually huge would take a very long time to move or delete.

Try turning off this feature, and waiting for the files to finish deleting, it may help to reboot between turning off the feature and removing the files.