Do you mean like a social networking system? Friendster is one example I can think of a site dedicated to it, but there are many out there.
Anyways, what I am getting at, is do you to simply have lists of who is friends of who? Or are you thinking of something more than just a list?
I think there was talk of having friends lists before, and we used to have something like that for the journals so you could share journals only with friends. However, that broke down when we upgraded the system.
I might not be the best person to comment on that though, as I didn't see too much of a point in such systems. It is nice to find out that two friends of yours are mutual friends and you didn't know that, or to find out about other cliques, but I don't think that would be as important for a site where most people know most other people (or stick to other means like IM and IRC).