Fast forward two months, and I decide to sign up for facebook.
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.
If you don't want to use the service, nobody says you have to. Facebook having your name and phone number doesn't actually mean anything until you sign up and so if you never do, the situation doesn't change.
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.