Earlier quoted context omitted.
Email is a tool whose value lies primarily in my ability to communicate with other users. Many people (including me) have several email addresses that we use for different purposes. That an email address can be used as an identity is something of an afterthought, and doesn't really fit into the "have multiple email accounts for different communication purposes" paradigm. My Facebook account is an identity first and f…
"My Facebook account is an identity first and foremost." Now imagine 5 years ago you said "My Friendster account is an identity first and foremost."
On the other hand, email is already known to be an unstable key for identity. And the "market" for identity providers is a lot more mature than it was 5 years ago. Besides, what if Friendster had established itself as a public identity provider 5 years ago? Maybe we would be using Friendster instead of Facebook today. Who knows.