So what if I wanted to use my Facebook contacts to see how many of my Facebook friends were on Buzz or Twitter?
Facebook wants us to use our Gmail username and password to import contacts from those services directly into their network but they don't want us importing our contacts into Twitter?
Because most of Facebook's users didn't sign up for this. They didn't sign up to necessarily share contact info. They signed up to connect socially. Do you expect twitter to start handing out email addresses of the people that sign up on Twitter.com? No because you sign up on Twitter to share short 140 character messages, not email addresses like you do when you sign up to use Google Contacts.
If a third party application wants your email addresses, you can go give it to them yourself. Otherwise what's the point of Facebook making it dead simple for people to give third party apps your email address? The only reason I can think is that greedy third party apps want to siphon Facebook's built up social graph by using spammy viral emailing techniques as opposed to getting individual users to sign up or add the Facebook application.
Also correct me if I'm wrong, but Facebook does let you compare email hashes. So this allows 3rd party apps/sites to link two users up if they are friends on Facebook.
So in the end yes it is convenient for Facebook to take this stance of not making it easy to export your friends' email addresses but it truly does have a legitimate negative aspect to it. And in the end Facebook still provides the requisite methods to make it easy for people to leave Facebook.
Facebook wants us to use our Gmail username and password to import contacts from those services directly into their network but they don't want us importing our contacts into Twitter?
I don't get how that is not a greedy thing.