usenet may be regarded as the world's largest conversational application, with over 17,000 newsgroups and 3 million users. Despite its ubiquity and popularity, however, we know little about the nature of the interactions it supports. This empirical paper investigates mass interaction in Usenet. We analyse over 2.15 million messages from 659,450 posters, collected from 500 newsgroups over 6 months. We first characterise mass interaction, presenting basic data about demographics, conversational strategies and interactivity. Using predictions from the common ground model of interaction, we next conduct causal modelling to determine relations between demographics, conversational strategies and interactivity. We find evidence for moderate conversational threading, but large participation inequalities in Usenet, with a small minority of participants posting a large proportion of messages. Contrary to the common ground model and "Netiquette" guidelines, we also find that "cross-posting" to external newsgroups is highly frequent. Our predictions about the effects of demographics on conversational strategy were largely confirmed, but we found disconfirming evidence about teh relations between conversational strategy and interactivity. Contrary to our expectations, both cross-posting and short messages promote interactivity. We conclude that in order to explain mass interaction, the common ground model must be modified to incorporate notions of weak ties and communication overload.