@Spoon: email isn’t a secure medium. If somebody intercepts the email, they have your password.
@JW: It’s a bit like this: would you give the keys to your house to somebody just so that they could get a number out of your address book? And you give your ATM pin to a random stranger so you could pay for book? What Facebook and sites like it who ask this kind of information from you are doing is just that. They should ask you for your friend’s email addresses, not your first born child.