There's no reason to assume that the old addresses in people's address books were the same ones Facebook had as primaries. If my co-worker was in my phone as tom@patientslikeme.com, but signed up for Facebook as tom@gmail.com, and Facebook tries to roll back? Data loss still, and possibly even a privacy leak.
I was basing this on the fact that the facebook app/sync would have pushed tom@gmail.com into the contact info in the same way that it is now in trouble for overwriting with @facebook.com.
Thanks for bringing this up as it would not work if they only push changes, and no change had been made facebook-side prior to the 'bug'. In that case, they should only rollback those that had been pushed out and the majority of email addresses would be lost. However, since users can go into the settings and restore their primary email address, it seems like it would be best for Facebook to encourage them to do so in the same way they've nagged me to switch to timeline forever.
