[jhartmann]

Depends on the cameras, but I would imagine Facebook and Google's tech + data to be able to positively identify 99% or more of the people. Especially with multiple shots per person. Accuracy might be as high as 99.9% if you have drones go over the crowds, and have high res cameras at chokepoints. Correlate this with cell phone data and probably they have 100% accuracy. There is a reason Google Fi exists.

I say all this as a happy Google User and a developer of Face Recognition Technology. Those in the know realized privacy has been dead for awhile now. Generally computers can do most anything we can do in say 200 ms or less with the proper training data, and they generally do it better than humans. The giants have all the data they need, they just need to spend time annotating training sets and running analysis over larger and larger sets until they reach super human performance.