The feature is now opt-in. It is turned off for all new users and for those who only had the "tag suggestion" setting on.
Disclaimer: I work at Facebook and this feature is managed by my team.
Good that he asked... we all should more.
One question, do you construct "templates" for faces that are unmatched by existing templates that are correlated?
This would allow a shadow face recognition profile to be generated that could be later matched to a known image.
I've had face recognition turned off anyway, and I'm not particularly concerned about it, I'm interested though in how internally FB manages "unknown" images, shadow profiles etc.