How we read online:
That's Jakob Nielsen's theory. He's a usability expert who writes an influential biweekly column on such topics as eye-tracking research, Web design errors, and banner blindness. (Links, btw, give a text more authority, making you more likely to stick around.)
Nielsen champions the idea of information foraging. Humans are informavores. On the Internet, we hunt for facts. In earlier days, when switching between sites was time-consuming, we tended to stay in one place and dig. Now we assess a site quickly, looking for an "information scent." We move on if there doesn't seem to be any food around.