For instance.. barrybe may be in a "Friends" circle and a "Programmers" circle. In a programmer to programmer relationship, I only want to hear relevant topics. As a friend though.. I may want to be subjected to your waves of pictures highlighting your 2nd childs 1st steps. There's no way to keep the kids crap out of my "Programmer" stream.
The same is true for your blog. Unless I can subscribe to particular tags, I can't keep our interaction focused. I may get a rant, a story, an educational post.. it could be anything.
So the next best thing is organize all content by topic (for me). And here we have it.. "Hacker News". Except.. this environment is far less personal and far more intimidating.
>But unless I use one medium for each audience, there will be overlap.
No, that's what hashtags on twitter (surely twitter clients do filtering??) and topics on blogs and groups on Facebook and search wherever is for.