They actually have - and make it a bit easier than G+ did - with Groups and Smart Lists. They also added a separate tier called 'Acquaintances' that you could

They actually have - and make it a bit easier than G+ did - with Groups and Smart Lists. They also added a separate tier called 'Acquaintances' that you could add people to a while back. However most users don't make use of it and will either share with Public or Friends. The only exception is things like groups - which are popular with things like neonatal classes and sports teams.