  Decentralized architecture offers a robust and flexible structure for online
platforms, since centralized moderation and computation can be easy to disrupt
with targeted attacks. However, a platform offering a decentralized
architecture does not guarantee that users will use it in a decentralized way,
and measuring the centralization of socio-technical networks is not an easy
task. In this paper we introduce a method of characterizing community influence
in terms of how many edges between communities would be disrupted by a
community's removal. Our approach provides a careful definition of
"centralization" appropriate in bipartite user-community socio-technical
networks, and demonstrates the inadequacy of more trivial methods for
interrogating centralization such as examining the distribution of community
sizes. We use this method to compare the structure of multiple socio-technical
platforms -- Mastodon, git code hosting servers, BitChute, Usenet, and Voat --
and find a range of structures, from interconnected but decentralized git
servers to an effectively centralized use of Mastodon servers, as well as
multiscale hybrid network structures of disconnected Voat subverses. As the
ecosystem of socio-technical platforms diversifies, it becomes critical to not
solely focus on the underlying technologies but also consider the structure of
how users interact through the technical infrastructure.
