Let's think of a typical use case where a "shadow profile" is useful. Let's say my friend Jane is on facebook. She has my phone number on her phone with my first name. Facebook notices that this phone number is not linked to any other accounts, therefore I must not be registered. Fast forward two months, and I decide to sign up for facebook. I put my phone number in (as a form of password recovery I believe) and it a…
And this is where your use case fails: some of us already know about Facebook and will never, ever sign up. Don't track us until we ask you to, full stop.