I know that Facebook uses the contact lists of your friends to suggests users to you [1], but I'm 99% sure Peter Norvig doesn't have my email address in his contact list, and that none of my Facebook friends are linked to him.
I think I've found another troublesome method that Facebook is using to suggest new friends.
A few years ago, I created a Google profile with a vanity URL [1] and a Facebook account with the same Gmail address. I never linked those two accounts, used third-party apps or imported contacts into Facebook. I recently created a Google+ profile and publicly circled some users when I suddenly noticed that those circled users started showing up in my Facebook account as "suggested friends". Those users don't follow me on Google+, aren't linked to any of my Facebook friends and they don't know my Gmail address.
I can't think of any other method used by Facebook to recommend those friends, except by crawling my Google+ circles. It's as easy as extracting my Google+ username from my Gmail address and scanning my circles at profiles.google.com/username.
I can't reproduce this with different accounts as vanity URLs aren't available for new Google accounts.
Could anyone with confirm this with their own Google profiles?
Do you think Facebook should be authorized to "scrap" contacts from other social networks, to extend build their own social graph about their users, without possible opt-out and no disclosure?