How about "I'm considering taking a new job on the other side of the city. Anybody have experience on the best cycling route to ...?"

Maybe the bike circle as a whole exists as a place to ask questions ("My chainset is hosed! Anyone have recommendations for a new one?") but he only actually goes out biking with people he doesn't work with. More to the point, why does it matter why he doesn't want to share something with them in this particular example when it's really easy to construct good examples?

There's even a simpler scenario then that.

I have a set of close friends that I share just about everything with. It's a somewhat large size. Some of them I am in fantasy sports leagues with, some of them are techie types, some of them are fans of the same sports teams I am. I don't necessarily want to flood the stream of people that I know aren't interested.

There's many cases where individual circles of friends have a intersections or just don't fully mesh. It'll be interesting to see how this proceeds. UI/UX is going to dictate how successful circles are.