"You can’t replace pants with shorts when your definition of shorts is: everyone buy pants and cut the legs off — pants will still be a viable business (the consumer is just altering the usage). Same too with Twitter, Facebook, et al, they are still relying on email for certain parts of their service (like adding new users or sending notifications) while wanting to replace email at the same time."
I hadn't really thought about the ubiquitousness of email from this angle before. Even the services that are trying to replace email as the primary communications channel require you to have an email address. Obviously, this is because a) today email is the way they are used to getting their communication of passwords, logins, etc., and b) you have to have a way to communicate to people before they start using your service.
But, it would be pretty ballsy for some new service to require either email or a phone number (or <insert your method of identity here>).
(via The Brooks Review)