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While political discussion may indeed be getting more polarized, (and that is arguable, read about Lincoln's time), it seems absurd to say the internets is causing it.
People may seek out like-minded people, but they don't need the internet to do it. Dittoheads are radio listeners, remember. People may choose to read Time, Newsweek, or other magazines that slant one way or another. And don't forget TV. You can choose to watch CNN only for Glenn Beck if you choose.
These are all different forms of media. The internet does better than all of these in providing facts and analysis from various sources. In fact, if more people would look up facts on their own using the internet and do their own analysis, they'd be better off.
You can read the comments section of any Salon articles and see that most people here are not sheep and have their own opinions.