Well, I'm sure social networks like Facebook have already an internal "how good a friend is this person to you?" index for each of your friends, based on how mu

Well, I'm sure social networks like Facebook have already an internal "how good a friend is this person to you?" index for each of your friends, based on how much you interact with them (private messages, invites to events, wall posts, likes, etc.) So adding an extra step to the friend organization process is probably too much work for most people, specially considering people are lazy to even put the context of a friend to begin with (remember that friend lists have kind of failed in terms of user adoption).