This doesn't really answer your question directly but I find a fun exercise is to take a picture of a face and modify it (clip it, skew, morph, rotate, repeat,

This doesn't really answer your question directly but I find a fun exercise is to take a picture of a face and modify it (clip it, skew, morph, rotate, repeat, etc), and then see at which point Facebook will cease to recognize a face in the image (and then perhaps your face specifically), while you still do. The real point is that: I believe there are some images in the search space that any recognition algorithm will not see a face in, but which your brain will and vice versa.