>I question the methodology behind this figure. The nature of the decentralized network makes it difficult to get good estimates. All I have are anecdotes. Yes, the overwhelming majority of users are on Mastodon. However, I personally interact with several users on several different Pleroma instances and a couple on GNUSocial instances, and I did not seek them out for this purpose.

However they are the exception. Because the overwhelming majority of users are on Mastodon, the Mastodon developers have the power to modify protocol implementations. If it was closer to an even split, any breaking changes would fracture the community, while in this case some smaller groups may be lost.