Yes they are. Feeds and the "more like this" will show your content. So similarly, you'll see content that is private and/or secret on a user by user basis. Other people cannot see the messages unless they are on the message stream/feed. To see this, when you @mention someone who is not on the message list, you will get a yellow notification bar at the bottom of your screen saying "X user will not see this content because they are not included in the message"