[yanw]

A friend won't care that you have their email address it's sorta expected, and one could also argue about whether an email address is even private information.

Or maybe Facebook could allow the export of some other friend identifier.

[Zaak]

If email addresses were rarely abused, then a private email address would be like an unlisted phone number.

Email address abuse is extremely common however, so public should not be the default.

[yanw]

But in this case emails are not being used to compile a list for spammers but to help someone find/invite their finding onto a new platform.

[kmavm]

Again, Facebook exports a unique identifier for each friend already, and has been doing so for years. This is the entire point of Facebook Connect.

[joe_the_user]

Indeed,

If, for example, you used the email of your friend to email them, you would have the address in your email program...

So Facebook is saying that it's OK that you could take the email address and put it in your email program and address book - but that automatically doing so is "just crossing the line"...