That's essentially what Google+ does. Twitter isn't designed for posting to different audiences, and Facebook won't let me show certain posts of my wall to certain people but not others.

Facebook does allow you to do that. It takes a bit of work to filter people out individually or put them in groups, or "circles" if you like. For example, some of my friends are excluded from all my facebook posts as a group by default. I include them only when I want to.

I think Google+'s problem might be that it takes too much work to curate circles. Most people might just settle for Facebook because you don't have to think!