Hmmm. I wonder if email could be the basis for a Facebook killer. The killer app for Facebook is distributing announcements and coordinating events.
I'm envisioning a federated announcement service that sends your updates to users on your or other instances OR email addresses. Non-users would still get the announcements and could easily redirect them to an account should they decide to join via a link embedded in each message.
(Yes, abuse becomes a problem. It may be fixable via an opt-in mechanism.)