Although much is known about how brokerage positions in social networks help individuals improve their own performance, we know little about the impact of brokers on those around them. Our study investigates brokerage as a public good. We focus on the positive and negative externalities of specific kinds of brokers: “hubs,” who act as the main interfaces between members of their own network community (“network neighbors”) and members of other communities. Because hubs access diverse knowledge and perspectives, they create positive externalities by providing novel ideas to their network neighbors. But hubs also generate negative externalities: extensive cross-community activity puts heavy demands on their attention and time, so that hubs may not provide strong commitment to their neighbors’ projects. Because of this, network neighbors experience different externalities from hubs depending on their own formal role in projects. We use insights from our fieldwork in the French television game show industry to illustrate the mechanisms at play, and we test our theory with archival data on this industry from 1995 to 2012. Results suggest that the positive externalities of hubs help their neighbors contribute to the success of projects when these neighbors hold creativity-focused roles; yet the negative externalities of hubs hinder their neighbors’ contributions when they hold efficiency-focused roles.