On the arc of social networks, I have a huge motorcycle forum and motorcyclists are pretty frisky. Over the years as we've created subforums we watched the arc go from hard-to-get-traction ==> positive network effects ==> awesomeness ==> it's getting hard to keep up and it's getting noisy in here ==> it's getting impersonal, who are these n00bs? ==> oh look, here come the spammers and trolls.
We would then slice the subforum into pieces. We'd get a few howls but 9 times out of 10 it would restore the pieces back to positive network effects. Some would stay small enough, like Old's Cool, a subforum for restoring vintage bikes, that it has stayed great for years.
Our hope was that by enabling the topic structure we have, people could create very specific topics, like Ural Patrol sidecars, to keep the signal to noise high.