However, I also think it's important to note that I am not a lawyer in privacy. I am a developer who wants to help out. I'll leave the legality and policy behind privacy to the professionals.
At the end of the day I would have absolutely no problem with my publicly accessible profile image being used in facial recognition in these circumstances. Imagine if I was a potential witness of something? Is it then OK to use facial recognition in that case?
And what else than "potential witness" will you identify? None of these pictures could be used as evidence.