Google+ "Circles" didn't really do this. As a poster I want to be able to say "this is a post about parenting" which is either a hint to the Algorithm which it can use in its quest to maximize engagement, or an explict signal to notify people who follow jefftk:parenting. With Circles, the best I could do was say "please show this post only to the friends who are in my 'parenting' Circle". This has two problems:

1. What topics someone is interested in is something they probably know more about than the poster does.

2. Because Circles were used for access control, someone I had not placed in the "parenting" Circle would not be allowed to read the post.

The closest thing I've seen to this is bloggers having tags, and making RSS feeds available both for everything and for individual tags.