We introduce the notion of a social catalyst in the context of social media a person who prompts interaction between their friends through their posts. We find not only that this activity is heavily skewed, but that it also correlates with participation in groups and events. We validate the behavioral measure using a survey wherein people nominate social catalysts in their friend network. Whether a person will be nominated as a social catalyst can be predicted using network features and the tendency of their posts to prompt discussion. Although individuals' role as social catalysts has not attracted much study to date, this paper demonstrates that it can be studied both via behavioral and survey measures. - View it on GitHub