Well, Facebook does allow you to download your profile[1], so Google could just write an importer for that.
[1]: http://tech.slashdot.org/story/10/10/07/1250242/Facebook-Imp...
[1]: http://tech.slashdot.org/story/10/10/07/1250242/Facebook-Imp...
I don't know exactly what's the file format from FB backup, but it might be possible to identify linked users based on similar content (as long as both users have already uploaded their profile to Google+).