1. Ideological in-groups have always been able to construct cocoons in which their own favorite sources take precedence over the mainstream media. The Internet may have made these cocoons more visible to outsiders, but it did not create them.
2. While the cocoon-builders have taken advantage of the Net et al, the chief effect of the new media has not been to reinforce the cocoons but to increase the likelihood that a stray signal will cross from one ideological tribe's territory to another, budging people from previous certainties and creating new cross-breeds. (Note: For our purposes here, the mainstream is just another tribe.)
3. No measure of epistemic closure is useful unless it looks at people's levels of closure over time. It isn't enough to see a group of Americans standing in one ideological territory with their hands over their ears shouting "I can't hear you!" at any given moment; you need to know where they were standing three years ago and where they'll be standing in three years' time.
4. All that said, if more people are indeed in ideological transit, one possible reaction among the people who aren't moving is to dig in their heels and yell louder. So it's possible that we'll see less closure among most Americans but more closure among the hardcore partisans who remain.