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 foremost. People do not typically have multiple Facebook accounts (a TOS violation). If that account goes away, that identity goes away.

You might make an argument that websites should allow you to aggregate multiple identities (Facebook, Google+, MS, Yahoo, etc) into a single account, or that there should be some sort of an identity provider that creates an aggregate identity across all those services. To an extent some of these websites are already doing something like this peer-to-peer - Facebook is an OpenID consumer, for example. Maybe sometime in the future this will be a big issue. But right now it isn't, and email-as-identity is already an annoying problem.