As part of the privacy changes, Facebook has enabled conditional access to profile information using friend lists and also using a category called “friends of friends”! For example, you can categorize all friends from work under a “work friends” list and then hide your online status from them. You can even hide a lot of other details from your “boss” friend list. The advantage here is that the “friend categorization” will be visible only to you!
Another feature is the “friend of friends”. Facebook assumes that these are your probable friends and you may want to share some details with them!
Now it is pretty clear that other services such as Orkut is going to embrace this soon. Orkut is already trying hard to bring out Orkut apps (Facebook strength is in its application platform with thousands of cool applications online).
This “friends list” feature is useful if you keep the lists to a minimum. To me the most useful list that I can create seems to be “family members” group to whom you can expose your contact details including phone numbers.