It is completely up to the author. When you post something you choose the set of circles you want to share the post with. One of the choices is 'public'. Ther

It is completely up to the author. When you post something you choose the set of circles you want to share the post with. One of the choices is 'public'. There isn't really a default for this. When you open the sharebox, the set of circles is pre-populated based on who you've shared with recently.