Honestly I feel like the only option is to just stop accepting new members at a certain point, because above a certain tolerance it stops being a community and becomes a city. CIP: I'm in a rural town for a couple days and everytime I walk outside everyone I pass says "Hi" or "Good Afternoon," the first time that happened I was kinda surprised I tried to figure out if I knew that person from somewhere, because where I come from (Chicago) strangers on the street ignore you, they don't wish you a good day and smile...
There's the whole "you can't know more than 150 people" study [1]. Which I think is very apt here, obviously we have to scale that idea (as anything less than like 5000 people means it doesn't exist when it comes to the web) but it might be a good idea to stop accepting new members or cut the community into chunks, if you wanna keep things from getting worse...
Finally, I've begun to really like OSC's version of the internet as presented in Ender's Game, at least when it comes to having real discussions.