"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).

[1] http://developers.facebook.com/policy/#policies

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.