Private and public content in blog, part 1.

First, the private content that is visible only to a owner of the blog is rather useless in my opinion.  Logs that are non-interactive have no or little value as far as the growing content of the blogs are concerned. 

But how about contents that you want to share only with the group of people?  The need for this semi-private contents usually arises not because the content is actually super secret but because it makes sure everyone who are in the group are speaking in a same context.  For example, if there is a web related course, the jargons such as "css", "doc type" and "coding" etc... would make sense without anyone explaining what each is.  The contents of the class can be viewable by public but can only be edited by group members or it can be entirely private if desired.  This group can be defined by user such as a friend group.

Another private content could be a private messages that can be discussed between two people.  Why include such messaging feature when there is a email and gmail can make it actually look like a conversation? Well... this also has to do with the context.  Instructor of course might want to give private feedback about an entry.  It makes more sense for that private messaging to be a part of the blog than emailing each other about the blog.

Memo feature where blog owner log whatever stuff he/she wants might also be desirable so that it separates the non-interactive contents from the blogs and still provide a way to leave messages to self for future references.

Public content can be anything that you want anyone to view and reply.  If broad participation or wide range of reaction is the aim of topic discussed, making the content public makes sense.