[aprescott]

A related study into how long people spend reading words on a web page found that on average people only read 20-28% of the text. For 1000 words, the average visit duration was less than 70 seconds.

http://www.useit.com/alertbox/percent-text-read.html

I read quite a lot of text online, and I certainly have a different style of reading a web page versus reading a book. I'd be fascinated to know how efficient scanning is after years of practice online, in terms of information consumed that you're able to recall. 28% is surely too low for adequate comprehension, though.