Facebook database design?

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.