Adding Facebook profiles to Address Book
You can copy the URL for your friend's profile on Facebook, and paste it into the Facebook profile field in the card editor.  Disregard the fact that the editor shows "User Name" placeholder text in this field; the URL works fine.  After saving the card, you can click on the Facebook label to the left of the URL, and you'll get a menu that includes "View Profile" and "View Photos".

I tested this on Lion both when using a profile URL, and when using a new Facebook username.  The "View Photos" action does not work when saving a profile URL, but both actions work when saving a Facebook username.