Online Literacy at Chronicle. One quote:
In the eye-tracking test, only one in six subjects read Web pages linearly, sentence by sentence. The rest jumped around chasing keywords, bullet points, visuals, and color and typeface variations. In another experiment on how people read e-newsletters, informational e-mail messages, and news feeds, Nielsen exclaimed, "'Reading' is not even the right word." The subjects usually read only the first two words in headlines, and they ignored the introductory sections. They wanted the "nut" and nothing else. A 2003 Nielsen warning asserted that a PDF file strikes users as a "content blob," and they won't read it unless they print it out. A "booklike" page on screen, it seems, turns them off and sends them away.
I think I agree that people do not read online material the same way they read books. However, that could be because so much online stuff is thinner in content than books.