Why would you want to keep the telephone number?
The telephone number is a good example of a situation where the technical factor prevailed over the human factor. Numbers are abstract and difficult to remember for most people. And since its invention we have needed to use lists to associate these numbers to things we actually can remember, such as names.
I think it will go completely the other way, and that in 50 years people will never have heard of phone numbers. The identifier will be the email address, and if I want to call someone I select that address and press "call", and a VOIP connection will be made. If I want to IM or mail, I press other buttons.
The email address is easy to remember, it has build-in identification of the purpose you want to use it for (private, business,
It's also a lot more profitable to say 'You don't actually want privacy' than fix the security and sharing model of facebook. If you don't expect privacy, all the various holes and dirty tricks no longer matter.
There, fixed that for you. Advertisers do not like privacy (of their viewers).
Variables don't; constants aren't.