If I'm honest, I'm exactly the same: I'm on HN right now to avoid my matlab coursework. While HN doesn't feel like it's designed for distraction in the same way FB and Twitter are, it does seem to have the same effect: there's a reliable, constant stream of content that's vaguely stimulating. I think this is the combination that's so addictive. It feels as though your brain enters a lower energy state where content is being fed into it with minimum effort. Something about the lack of required energy to find and consume content seems to be what makes it powerful.