Google photos has the advantage of a limited search space. Any photo you take is overwhelmingly likely to be one of the few faces already in the library. Not to say facebook couldn't solve the problem. But the ability of Google to do facial recognition with such poor inputs is that it's searching on 40~ faces rather than x billion faces.

Can confirm, have seen Google photos misidentify strangers. I'm sure better technology exists, but Google's system has weaknesses.