Geoffrey Nunberg considers how technology tracks them:
Think of the tens of thousands of used copies of Pride and Prejudice still in circulation with "It is a truth universally acknowledged …" highlighted or underlined and "IRONY" written in the margin, as readers affirm that they're in on the game. … The opening sentence of Pride and Prejudice comes in second place [on Amazon's list of the most highlighted passages ever], just behind one from The Hunger Games, whose selections occupy 13 of the top 15 slots. As Ann Blair has said, the annotations made by ordinary readers have always been useful windows on shared patterns of thought, and those of us anxious about the fate of literary culture can take some comfort in knowing that Austen can still claim so high a place in Amazon's collective florilegium. Still, you might wonder if it's possible to grasp the irony of her sentence without undermining one's faith in the wisdom of crowds.