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This is probably also a really clever use of captcha as a way to train their face-recognition algorithm.

They already have the faces annotated via the tagging system. To be really sneaky they could inject faces which have been detected but NOT tagged. However they probably have enough data anyway - enough being the largest facial recognition training data set in the world.

The tagging made by users in their photos and face recognition is different.

When a user has to tag someone they are given standard sized box to drag over the person you want to tag while the face recognition systems selects only the face of the person no matter how big or small it is on the picture.

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