Even if you take the time to create lists for your different social groups, these groups will evolve over time, because of new friendships, fights or whatever; which means that you need to spend time maintaining who belongs to what group. That will generally not happen.
The other solution is to rely on intelligent systems that manage your groups (Facebook could do that kind of things based on who interacts with whom, and what kind of subjects, etc.) or who sees what (Facebook already kind of does that from the point of view of showing you what you will find interesting, not what other can or not see at all) but relying completely on systems like that is difficult and scary, and will still hinder sharing stuff blindly.
things coming and going should be a feature not a flaw of the way people interact online...