Jakob Nielsen pulls together data from eyetracking about how little people actually read on the Web. Its mostly scanning I know, but it is worse than you think:
' .. On the average Web page, users have time to read at most 28% of the words during an average visit; 20% is more likely. We've known since our first studies of how users read on the Web that they typically don't read very much. Scanning text is an extremely common behavior for higher-literacy users; our recent eyetracking studies further validate this finding ... '.
Is it because there is too much information instantly available now? I scan books sometimes but it is usually because I am looking for something specific. I scan the Web because I am looking for something interesting ... then it is off on another link. The article has some interesting links on the subject, but you have probably stopped reading this by now.