A "tipping point" in other words. That seems plausible. I think the problem with simply blocking people and sites is that the site still "tips", and you end up ignoring too many people. The happiest people are those who dont' have to ignore anyone.

Which brings up another point - do you really want to foster debate? Maybe it's better to just feed people what they want. If I can't have thoughtful, considerate, and informed debate, I think I'd rather just have the echo chamber than impolite, rude, paranoid extremist rants.