Really. I think social networks - ALL social networks - need a topic/channel/directory feature.
No, tags don't work, nor do circles or list. Google+ and/or Facebook pages work somewhat better, but they're designed for a different purpose - posting on the behalf of 'entities'.
Let me explain: I am a human, my name is Alan Franzoni, and I sometimes want to share with the world my thoughts about something. I often discuss software development, and I've got a Twitter account and a blog which I've setup for discussing such topic; there I write in English for a worldwide audience. I sometimes go a bit astray, but I try to keep mostly focused on such topics there; I've got a separate blog where I collect Linux/OSX/Unix related tips and tricks that don't really fit in the wider software development audience.
I've got a Facebook account as well, where I write in Italian, which I mostly use