Back in the day you could, given somebody's UNIX account (email address), type finger firstname.lastname@example.org and get some information about that person, whatever they wanted to share: perhaps their office location, phone number, URL, current activities, etc.
The finger protocol, sadly, died.
Fast-forward to Web 2.0. We're currently bickering about how we do interop between all these social web services, and even how we represent a person's identity. The two main identity identifier camps are email addresses and URLs.