I have signed up to yahoo in Feb. 2011 once and never used their service again, not even yahoo.com. There are zero personal details shared with yahoo. Today I signed up to Flickr.com using Login with Yahoo (private browsing session) and it clearly showed many of my friends on the front-page for me. I never used flickr until this day, so how have they obtained my personal friend graph? Isn't that technically illegal (I live in Europe)?

The Yahoo account was never connected to my Twitter account or anything else. It's a pure yahoo account, with no relation to existing email addresses.