I have always wondered how Facebook designed the friend <-> user relation.

I figure the user table is something like this:

 user_email PK
 user_id PK
 password

I figure the table with user's data (sex, age etc connected via user email I would assume).

How does it connect all the friends to this user?

Something like this?

 user_id
 friend_id_1
 friend_id_2
 friend_id_3
 friend_id_N

Probably not. Because the number of users is unknown and will expand.