[amarcus]

FB's recommendation engine looks at the contacts shared by their users. If they see you in a friends address book, it will recommend them to you. I think it looks at name, location, email, cell etc... to make the connections.

As well as that, even though your friend is using a different email address to the one he uses to login to FB, I bet you that FB knows your friends other email and has made the connection.

Here is a fun little game to play - Create a brand new FB account using a brand new email and your actual cell number. The list of recommendations that FB will give you are people who have your cell number in their phone's contacts application. Really cool way of knowing who has your number.