Facebook friend suggestions.. how do they know?

[Persieee]

You know on the right hand corner of the facebook homepage, theres a friend suggestion box? Well how does Facebook know who I know? Sometimes there would be people from my school (network) or even through my other friends, they would be suggested there, I understand that. But, there was this kid who I haven't talked to in over 5 years, he's my dads friends son, so we're not that close, but anywho he's from Europe and I'm from the US, and well, I just liked to know how does Facebook know that I know this person?

I've never searched him on facebook, I totally forgot about him he probably forgot about me to, I've emailed him through my e-mail once probably 3 years ago. and well, let's just say we're more of acquantinces not particulary "friends".
no one on my friends list but me, plus we dont even go to school together he's from europe and i'm from the states.