Google+ had circles as well. I found it very hard to properly segment people properly. There are rarely such hard boundaries on relationships. If I'm snowboarding or barbecuing, it's very seldom I only wanna share that with just friends, just family, or just co-workers. It will usually be much more interest-based.

Trying to create interested-based circles was also hard, as the cardinality explodes quite fast (and you're back to square one with not knowing who you share what with).

I guess this is where Snapchat is onto something with giving a very rapid selection of who you want to share with. I usually have a small group of people in mind when I want to share something.

Defining that group, fast, almost on a per share level is the key challenge.

I think the "circles"-approach could be a way, but I think it must be much more dynamic and alive than how Google+ solved it. It must continuously evolve.