Facebook prides itself on getting each user to have only one account, with their real name, that really reflects who they really are. Tech Crunch head honcho Michael Arrington has demonstrated how easy it is to set up fake accounts on Facebook. He created an account for Google CEO Eric Schmidt using an email address for Schmidt that was easily found online.
Of course I could have created a fake Eric Schmidt account without using his real email. But by using that email address Facebook immediately started suggesting friends to me - presumably people who have uploaded their contacts, including that email address, to Facebook in the past.
He skipped the confirmation part that comes after registration, when Facebook sends you an email, because he obviously didn't have access to the email. Even so, he was able to make and accept friend requests (including the head of YouTube).
Facebook could make all activity hinge on that email confirmation but to do so would be to slow down the thrill ride into Facebookland and make it less appealing as a result. So they don't.
Obviously, this account has probably already been deleted. But that's because Schmidt is famous and Arrington wrote about it and people like me picked it up. But what's to stop someone from doing the same thing with your account and your identity?