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“The Internet is making us stupid” which was probably not written by Sunstein but by someone at Salon, as a way of throwing blood in the water.
As pointed out by Augo Knoke at 6:10 am, the basic ideas, that humans surround themselves with like-minded people and ideas, and that groups of like minded-people usually come up with more extreme positions were demonstrated in social psychology experiments 40-50 years ago.
So, people acted like this long before the internet.
I agree with Tigerr at 1:42 who says “Sure, the internets will amplify it in a way, but it's definitely not the cause. We are.”
Modern communications, ready access to information, propaganda, diatribes, etc. do allow people to wallow in their own pre-conceptions. Quick thoughtless, emotionally driven emails proclaiming one’s heartfelt solidarity or disagreement provides an anchor that publicly links oneself to a position, and limits further consideration, regardless of one’s side of the political spectrum. It’s much, much easier to do this today, with the internet (a neutral tool) than it was in 1850 or 1950 or 1980.