Facebook started to deteriorate when they made it even harder to group friends. Before it was really simple to assign someone to a certain friend-group at the same time you accept their friend request, this functionality appears to have been removed.
Facebook would be vastly better if the interface was simply more group oriented. If grouping friends as you add them was front and center. A group list, where each friend-group element contains links to functionality such as: what you share with this group, news feed\number of new posts of this group, number online to chat in this group.
Would be vastly better than having a list of groups for newsfeed, list of groups tucked away under privacy settings, list of groups tucked away in chat box. Friend Groups are parents of features rather than Features being parents of friend groups. It just plain psychologically works better.
I think facebook is a genuinely useful site compromised by a shit interface. The facebook interface is like urban sprawl: an uncontrolled expansion that ocassionally gets band-aids rather than a centralized plan based on User Experience.