In Narrative all posts are public. There is no concept of private posts. Everything you post is out there to be seen.
Now, as to which Niches you're following, that's different, and is part of your Member Profile. Right now there is no way for anyone to view what Niches another individual is following, what other Members they may be following. As we move along though, visibility of some of those things will become options that you as a Member will to able to decide. You'll be able to allow others to view what you follow, you'll be able to allow others to see who follows you and who you follow.
You can refer to the spec and see what elements of your profile are public, what is private, and what is your choice under Member Profile. The profile elements and then the settings are discussed in that link.