It's not that simple. There are many many ways that your information can end up being referenced in an identity graph and be discoverable by Facebook that don't involve signing up for Facebook at all. It's more - don't sign into a public wifi network, don't hand over your email address at a checkout, don't sign up for a store card.. oh, and don't sign up for Facebook either.
Not just that, but even if your number is not shared with Facebook, but Facebook knows that ten of your friends all contact you three times a week, you're still in the graph even if you're not personally receiving advertising.
That practice can be fought more easily in courts. Also the more user deleting they're accounts the less "friends" you have that share your number with facebook.
