"III. Storing and Using Data You Receive From Us
...
7. You must not use user data you receive from us or collect through running an ad, including information you derive from your targeting criteria, for any purpose off of Facebook, without user consent." [1]
I suspect that means you need consent from every user whose information you collected (your own list of contacts is made up of other peoples' basic information).
From a more thorough read of the policy, III. 3. looks like it means that you're ok to use "basic information" from someone who connects to your service however you like, as long as you don't go selling it to ad networks/third parties. That means that you can store one person's first name, last name, email address, profile pic, FB ids of friends - but not any of their friend's details other than their id.
It's not much, but it's enough to help create someone's account on your competing social network. Whether you'd be able to store/export their posts/other people's comments is a different question - it depends on the definition of application, but FriendFeed seemed to be able to do it, so why can't someone else?
You might be able to take a user's data to establish their profile, they consented by using your application after all. But I don't think you can export their friend list and use it to invite them to your new social network.
Either way, I think Facebook could find something in their ToS to shut you down if they feel like you're stealing their users away. There was a recent article about a tool from Power Ventures that allowed you to login to multiple social networks and aggregate the messages, friends lists and what not. Facebook seemed to claim it was a violation of the ToS because they were accessing account data using "automated means."
It's generally within FB's terms of service to store Facebook ids, but unless you're a larger company like Yahoo, I don't think you're allowed to do much else other than use the data for your app.
I'd wager than any such app that could help users migrate to a competing social network would be shutdown extremely fast.
Most users migrating their emails to another provider can do it through a desktop email client. At most it might require learning how to download an mbox file or some such. Pretty hard stuff for an average user, but not impossible.
Try telling a user that they'd have to program their own app on a proprietary third-party API and host it on their own servers and go through some sort of vetting process just to get a backup of their emails and... well I hope you can see where I am going.
