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
I would guess the only thing they can't undo would be the time that has passed. If any data Facebook ever had on any of their users (such as their primary email address [edit: at any point in time]) was ever lost, they wouldn't have as much to sell to their customers!
Read me blathering on further about this: http://news.ycombinator.com/item?id=4192299
This is certainly within the realm of possibility and indicates that you hold the team's technical ability in high regard. If it is indeed true, I would wonder what percentage of the folks involved in the implementation actually thought about it and how high/wide the issue was escalated. I have certainly been in situations where after-the-fact I found out that something I had been a part of shipped with known bugs/features I didn't know about that I would have raised a lot more concern over.
If you've never worked someplace where they have dedicated staff for source control, that fact alone might give you an idea at the level of expertise involved here. It's nobody's first time at the rodeo, and setting the sync API on the change (as described below) wasn't just slipped in by an intern.
You finding out after-the-fact and still having an opinion about bugs shipping in code you contributed to only reinforces my point: someone made the decision to go ahead anyway.
Rolling back the change won't get people's time or email back, or undo the damage to facebook's reputation.
As I pointed out elsewhere, they can just forward the email to the intended address as an unwelcome MTA, so I disagree with your statement that they can't get people's email back. I agree on the time and reputation points though. However, I don't think any rollback has ever gotten those back, so 'rollback impossible' because it can't do what any rollback has ever done seems disingenuous.
Just restore from your last iCloud or Device backup.