@celia So, yeah and nah.
I was on G+ and am familiar with Circles. They had their uses but were mostly a pain.
What's missing is the ability to target a toot at a specific group of people, other than 1) all your followers (which is wide open for unlocked profiles) or 2) a specifically listed set of accounts.
G+ Circles were OK at directing where content went, but abysmal at filtering incoming content (resulting in many "you're holding it wrong" arguments).
And management was an utter PITA. I ended up nuking most of mine. There were a few useful features such as being able to permit Notifications, Comments, and Stream for only specified Circles. (I made heavy use of this, with three circles named after those permissions.)
G+ Communities addressed many of those issues, though they also had many, many warts. Smaller and private communities of 50--100 or so members were actually highly useful.
G+ Collections were quite good for organising your own content, and allowed others to subscribe or mute specific topics. Others have mentioned liking those, I certainly did.
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