1. Phone number: in theoretical terms it works as an identifier.
However there have been attempts to get it used as an alternative to an
email address and there's been no traction.
And BTW, phone number only sometimes -- maybe
even often, but not always -- maps to a specific individual...
How many apps do you use on a phone or tablet? Lots and lots of them
use your mobile phone number as the identifier, since everyone knows
his own number, and mobile phones are quite personal. Of course, bad
programmers are everywhere, and since my tablet has no phone number,
I'm certainly not saying it's a great idea, but it's got plenty of
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