I disagree, I think we should demand that `username@domain.com` (coupled with a digital signature) be the one true way (TM) that services treat identity. Wheth

I disagree, I think we should demand that `username@domain.com` (coupled with a digital signature) be the one true way (TM) that services treat identity. Whether domain.com supports an email service, an HTTP service, or a social service is secondary. Standardized representation of the principal is the primary concern. Twitter and Facebook represent the principal using their own alternative formats to assert ownership over the domain, not because they're good formats (@foo, fb.com/you). They've normalized the idea that the textual representation of the principal should be different between different services, but there's no fundamental reason it should be so.