Open vs. Closed Instances. Mastodon instances can be separated into two groups: (i) open, allowing anybody to register (47.8% of instances); and (ii) closed, requiring an explicit invitation from the administrator (52.2%).
Figure 2 presents a CDF of the number of users and toots per-instance, separated into open and closed instances. Overall, the user population distribution is highly skewed, with distinct traits of natural centralisation.
For both kinds of instances (open and closed) the top 5% of all instances have 90.6% of all users. Similar patterns can be seen among toot generation, with the top 5% of instances accumulating 94.8% of toots. This of course confirms that, despite its decentralisation, Mastodon does not escape the power law trends observed in other social platforms [49, 10, 16, 27].
Figure 2: Dissecting Instances with open and closed (invite-only) registrations. (a) Distribution of number of toots and users per-instance (b) Number of instances, toots and users for open and closed registrations; (c) Distribution of active users (max percentage of users logged-in in a week per instance) across all instances.