Poor Man's Email

Google CEO Eric Schmidt describes Twitter as a poor man's email system.

"In other words, they have aspects of an email system, but they don't have a full offering. To me, the question about companies like Twitter is: Do they fundamentally evolve as sort of a note phenomenon, or do they fundamentally evolve to have storage, revocation, identity, and all the other aspects that traditional email systems have? Or do email systems themselves broaden what they do to take on some of that characteristic?"

It seems like most of these systems are doomed on some level to reimplement all the basic features of SMTP. Abstracted a bit, it's as if everyone had their own personal mailing list and anyone else can subscribe.