Wrong: Our attention spans are hopelessly on the fritz.
Right: The internet has brought our world more information choices. Sure, we give the average choice less attention because it’s competing with a larger number of alternatives. But we abandon reading one newspaper article not because it bores us to death but because an alternative article in some alternative publication presents itself as more interesting.
So we may read less of your newspaper article before we decide that another one looks better. The switch results from a marginal cost-benefit judgment between alternatives, not from a stunted conclusion that whatever in front of us is beneath us.
In other words, a fancy counterfactual: Imagine a possible world much like the one in which you posit that people still have healthy attention spans—a world circa 1958, for example, fifty years ago. Now imagine that your possible is world is different from the actual 1958-world only insofar as the people who inhabit it have as many (analog) sources of information at their fingertips as we do (analog and digital) sources of information. I claim that the people in your world give their average information choice about as much attention, not much more and not much less, as we do ours in our actual 2008-world.
Thanks to Jeff Jarvis for inspiring this post.