And even then there are levels of gray - I registered on Facebook in 2007 with my real phone number. I changed providers soon after, and no longer have that num

And even then there are levels of gray - I registered on Facebook in 2007 with my real phone number. I changed providers soon after, and no longer have that number. But it's still the number Facebook have for me. I now (and many of my friends) have a pay-as-you-go SIM for anything (including parcel delivery) that want a number.