Do online political discussions tend to aggregate diverse voices in cross-cutting debate and deliberation? Or do “audiences” for online discussion tend to fragment into ideological echo chambers? In the wilds of threaded discussion on the internet (as opposed to deliberative polls, moderated discussions, and other designed venues of deliberation), networks of political discourse emerge from billions of individual choices by millions of individual citizens about what to discuss online, where to discuss it, and with whom. Do these choices lead individuals to interact across ideological divides, or to cluster within them? How we understand the value of internet-based forms of political discourse is in part dependent on this question.