The pieces are all there: Crawl Facebook's publicly available data and index people's profile pictures. Then, provide a service where anybody can upload a picture and it will attempt to provide a match for the faces in the picture. It's sort of a DNA matching database for photos, if you will.
The implications are a bit frightening, but it's likely just a matter of time before someone does this exact thing, unless the law changes. IANAL, but it seems pretty clear that there is no expectation of privacy in public.
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