Would applying such a 'rollback' involve replacing facebook.com email addresses with the original external domain addresses in the contacts list on the devices affected by the write-over issue?
If so: Facebook have taken private data and placed it in a central location
Yes, a (hypothetical) "complete rollback" would involve replacing removed data. Sure, data privacy would be a concern, but it's beside the point that rolling this back would necessitate un-overwriting.
If so: Facebook have taken private data and placed it in a central location
If not: it isn't a rollback that I understand
Disclaimer: I'm not a programmer