Can't you add a button (either “Join Diaspora to Friend me” for non-members, or “BeFriend on Diaspora” for non-connected members, or “UnFriend of Diaspora” otherwise) on each Diaspora-Profile page? That would allow users to switch their relations by hand to another Graph, presumably distributed —— a lengthy process, but one that would kick-start the transfer, and could be accelerated with a “BeFriend all Fb-Friends with Diaspora accounts” button later on?

The private Events is a very good exemple of why cross-platform scuks, but I don't see any other option than trying anyway.