US bills have serial numbers, which could be used to track people but AFAIK, no one has made this simple. In a world where the government insists on tracking its citizens, I could get rich if I built a device that could track serial numbers and fit inside of vending machines. If most cash registers had such capability, cash wouldn't be so anonymous. I think banks do track serial numbers that come and go so this kinda can be done? Am I wrong?

Let's see the internals of what Facebook's code is doing with our data and the decisions it makes. Transparency is useless to me without seeing the blue prints. But sadly, I'll still use it to connect with friends.