I also remember this slide deck from some hip product guy who pitched circles.. 150 slide hot mess.[1]
[1] http://www.businessinsider.com/heres-the-presentation-that-i...
In real life, you get "circles" for free! Whoever you are with in the current moment is your circle, and only they have access to what you "share" during that time period.
There is some leaking through the grapevine, but we have an intution for that and usually handle it decently well. It's often even desired.
And the whole reason Facebook et al are popular is that in real life it's really hard to get all the people you want in the right "circles" (i.e. places) at the right times.
You can send a group message, which I suppose can sort of function like a circle, especially if it's long lived. But it would hard to manage many of those.
Instead we really should use NLP to automatically categorize contents.
e.g. I wanna make my 50% for seriously thought-provoking articles, 20% for shitposts, 20% for news and 10% for memes.