I suppose this might put an end to the Diaspora dream.
Some people are totally comfortable with the risk, but frankly I think it would be good for everybody that the "social networking infrastructure" is not owned by one company. Then... like email, ISPs, IM, mobile phone providers... everyone would have a choice and companies could compete for their customers.
Now, I can see ISPs setting up social network nodes for their users, I just wonder if there won't be some similar pain if/when one changes provider.
Not true for all the contacts in your fb profile.
"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).
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.
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?
http://blog.friendfeed.com/2009/08/friendfeed-accepts-facebo...
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."
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.