I'm currently reading Don't Make Me Think: A Common Sense Approach to Web Usability by Steve Krug. (You'd be surprised at how many people see this book sitting on my desk, laugh, and say, "Don't make me think?! What's that?")
It's a really great and rather slim book on how we use the Web. A lot of the ideas are probably things you've subconsciously thought before, but with this book, Krug makes sure you consciously think about them. The section I've enjoyed most so far is one on Web site navigation. Krug says "the Web experience is missing many of the cues we've relied on all our lives to negotiate spaces," like sense of space, direction, and location. "On the Web," he writes, "your feet never touch the ground; instead, you make your way around by clicking on links."
"When we want to return to something on a Web site, instead of relying on a physical sense of where it is we have to remember where it is in the conceptual hierarchy and retrace our steps. This is one reason why bookmarks -- stored personal shortcuts -- are so important, and why the Back button accounts for somewhere between 30 and 40 percent of all Web clicks."
"This lack of physicality is both good and bad. On the plus side, the sense of weightlessness can be exhilarating, and partly explains why it's so easy to lose track of time on the Web -- the same as when we're 'lost' in a good book."
So there you have it. That's the reason we can spend hours online and not realize we've been sitting in front of the computer for so long. And who knew the Back button was one of the most used buttons on the internet?