AHA! So it's not just me:
I find that the more I read online, the less I read off. I don't think it's even a matter of using up my reading time. It actually destroys brain cells or something, because if I've been doing too much online reading, I lose the patience for following a sustained or subtle argument, or reading a complex novel. One of my reasons for frequent blogging disappearances is recovery: I need to get away from the fast and facile and let my brain heal. It actually feels like recovering a bit of humanity that I forgot I had.
As a fellow sufferer, lemme tell ya, the phenomenon that Jeanne D'arc is describing up there is real, and more than a little worrisome when you first notice it. It just feels so ... organic, somehow, like you've damaged a part of the brain itself. Fortunately, though, Jeanne right about the second part, as well: blog breaks help, and, in my experience, anyway, long blog breaks -- the kind measured in months rather than days -- are completely restorative. (That said, you probably shouldn't rely on my experience with blog breaks. After all -- and as regular readers are no doubt thinking to themselves at this very moment -- some of us may simply have less to restore than others.)
NOTE: Via Kevin Drum, who rightly calls this vexing malady "a real problem."