1) self moderation: Google Plus has circles which you can theoretically use to share specific topics only with circles that you know to be passionate about them. Requires manual work to setup the right circles and people rarely do it.
2) manual moderation: Hacker News uses pg's time and some admin/moderator supervision to trim unwanted topics. Requires manual work which expands proportionally with the community's size if no automation is used.
3) automatic moderation: HN's flag system and community moderation (voting, Slashdot karma) seem to work relatively well, but they've been used until now predominantly to "rank" good comments to the top as opposed to "clustering" online communities into "sub-reddits" with focused interest groups.
Sounds wrong, doesn't it? The circles sharing model is backwards-- why should I have to make guesses about what other people are interested about?
I've written about this on a previous App.net HN thread (http://news.ycombinator.com/item?id=4304061, core excerpt = "I want to subscribe to that person on a "coding" stream, and leave the "personal" stream alone. People are multi-dimensional beings with many orthogonal interests; Consider giving them multiple stdouts.") and App.net PM said it was duly noted. I.e. it will be ignored, and the site is destined to be a noisy mess just like everything else that currently exists, except it will also cost money.
Subjot (http://subjot.com/) would allow you to subscribe to people based on topic, but they shut down recently.