> Personally, I find that Facebook has become my address book

Before disappearing into outer space (leaving Facebook) I found that Facebook had become my address book.

I think that is Facebook's greatest use: a self-updating address book. Instead of all my friends having to update their address books when I make a change, I should update my contact info and the changes should automatically be available to them. It's a great idea. Now, we just have to make it happen outside of a closed system.