Being a teacher, I somewhat agree that the internet is making us stupid. I saw this most clearly in trying to get my students to absorb information on climate change and make an argument for or against specific premises in the debate using a PowerPoint-style presentation. Some of them excelled, others just wanted to cull text from websites, copy and paste them to slides, and read it all off the wall to the bored audience. They had no sense of how to analyze the information, distill it into points, and support or refute those points using evidence and logic.
On the other hand, I don't know how I feel about the generalization that reading on the web in some way detracts from the cognitive skills acquired from reading text in print. I do both, and while I know that my attention span has shrunk, the content on the web is so much more distilled and comprehensive if you look on the right sites. For example, I read Al Gore's The Assault on Reason, which really went into detail in the first few chapters arguing that TV has fundamentally changed our "gut level" reaction to politics, but all of that could've been summarized just as easily in a blog post with links to scientific studies that support the ideas.
I think the web is great for counter-acting the very problem Gore decried because we have to process things cognitively in reading them online, rather than just reacting to a visual image, in reading politics blogs. This is good, although we now filter our news sources to our liking, and so supposedly drive partisan wedges ever deeper in our political divide, in addition to increasing our ignorance of viewpoints that we selectively exclude. The reason this is good despite the costs is that most people simply weren't armed with factual support and logical arguments to justify their political preferences. Partisanship may have increased, but at least most political partisans are well-educated (or misinformed) in their strong opinions, rather than going on the basis of what I heard my grandmother say about candidates: "I like the look of him."
Long story short, heavy web users know how to go to reliable sites for valid information, go through and distill long sections of prose into key points, and learn how to analyze writing and presentation for any viewpoint bias. I think that reading books is fantastic, and ought never end, but the power of the footnote is impotent compared with the power of the hyperlink to propel you to reference information. How many times, when you're reading a book, do you remember some trivia apropos to the topic and wish you had that at your fingertips? That's part of the sexiness of moving away from dead tree versions of information.
