[cbhl]

Identifying people's faces, especially caucasians, is relatively considered straightforward nowadays. Google spent a lot of time working on the problem so they could obscure faces in Google Street View. You start by looking for the eyes, which are dark spots surrounded by white. Then, you go from the eyes to the nose.

Identifying which person a face belongs to is a classification problem, which gets difficult the larger the set you're comparing to. Facebook has some tech that is interesting here, but it also only checks your friends and some level of false positives is acceptable. I imagine it that what algorithms they have aren't quite good enough to, say, identify your face out of an entire city's worth of driver's license photos. (Yet, which is what this article alludes to.)