We don't have the opportunity to post things privately for two reasons: 1. We can't think of anything that you would be doing on this site that you should feel the need to hide, and 2. It's not a trivial thing to put in and we felt our time was better spent working on core features of the site. It may be implemented eventually if there is a large demand, but we don't see why it should be necessary. If you have a specific issue and would like to block access to your information from somebody, please contact us.