Auto-generated description: A comparison chart shows diversity and concentration metrics for Fediverse and Atmosphere platforms, displaying server counts, sizes, and Shannon Index values as of August 30, 2025.

One of the things that I see on repeat in discussions around federated social networks is how decentralised Bluesky is compared with, say, Mastodon. What I like about this site is that a) it’s visual, and b) it tries to use some kind of scientific rationale to compare the two.

So yes, while you can say that Bluesky is decentralised in theory, in practice it’s very much not. Yet, anyway.

This site currently measures the concentration of user data for active users: in the Fediverse, this data is on servers (also known as instances); in the Atmosphere, it is on the PDSes that host users' data repos. All PDSes run by the company Bluesky Social PBC are aggregated in this dataset, since they are under the control of a single entity. Similarly, mastodon.social and mastodon.online are combined as they are run by the same company.

A note about the measurement:

The Shannon Index is an entropy-based measure used in ecological studies. It is computed the same as Shannon entropy using the natural log: the negative sum over all servers of the “market share” times the log of the market share. Lower values indicate lower entropy (a high concentration of one species), while higher values indicate a more even population. In this context, the maximum value is the number of servers, which would mean that all servers have equal population.

Source: Are We Decentralized Yet?