How does Facebook know friends?

[Howard]

My brother recently joined Facebook.He hasn't added any information or added any friends but he has been asked if he knows certain people (who are his friends) so they can be added as friends. How does Facebook know his friends if he hasn't given any information or added any friends?
Still confused. My brother hasn't added me as a friend so how can my email contacts make any difference?
Has his email address book be hacked by Facebook?

[Justin]

Through his e-mail contacts most likely.

[john]

There are two ways:
1. Your location
2. Your email contacts.

And of course, when he starts adding people, mutual friends.