It is, but I think it's tough to do well. Google+ implements this thing where you tell it how much of each circle to show, but then you have to do these weights

It is, but I think it's tough to do well. Google+ implements this thing where you tell it how much of each circle to show, but then you have to do these weights up front. Ideally system would try to figure out what you like by measuring time spent reading threads, upvotes, link clicks, etc and automatically set those weights for you. This doesn't preclude giving users the options manually setting weights, which could be another useful signal.