When I signed up for LinkedIn, I put in my name and "work" email address and straight away, without me having provided any other piece of information it was suggesting people I may know. It was not possible that it could have shadow guessed this from my email since those colleagues don't have that one, but both my GMail accounts are linked so I wonder if that was the weak point. Definitely something strange going on and it scared me somewhat...
I should also note that at no point did I use the "find my friends" type feature and give it access to my GMail to mine my contacts either.