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In the UK the press has a long and glorious history of each paper being the one great truth to its readership. You have the Times, and you have the Guardian. And right up there on the Guardian's masthead is a long diatribe about socialism, the worker's rights blah blah blah. I doubt they even consider themselves a news organ anymore and self label themselves as a vanguard of opinion. You know when you open the Times or the Guardian or the Independent what you're going to get. Same thing in France or Italy with their famous Communist newspapers. There is no news anymore there hasn't been for a very long time. There are only certain fact-like attributes spun in such a way to support wholeheartedly the official line, whatever it is. And the readership is thrilled with it.
I think the big hole in Sunstein's assertion is not that it's real or apparent but that by implication the Internet was going to erase this hardcore self reinforcing narrowminded bias. Do we imagine that the collective readership of Salon would materially change were there no Salon? They'd still hold the same beliefs. What the internet does is allow everyone to grouse about it to a bunch of people who all feel the same way. It's like a 12-Step program without the happiness or hope.