As I pointed out in other comments, something like FB can never be truly open for the simple reason that most people really don't want their posts to be searchable by everybody. Facebook is Facebook because you can post silly things on it without worrying if some future employer could find them if they wanted to. Sometimes an information silo can actually be a good thing, I suppose. But perhaps more research into federated social networks could take away this concern.
Theoretically, Google could let you "hide" certain posts if you're not logged in.
The thing that killed blogs as they are is that they're too serious. When signing up you need a title, subtitle, and input is optimized for long essays. Facebook is optimized for sign up, write your name, find friends, and post pictures, videos and sometimes text.
Really, if Google+ was Blogger Basic where you sign up, put AdWords, post pics, and it could have taken off.
