The number of users by instance in #mastodon is highly unequal. Some servers (mastodon.social) have a significant fraction of the number of users. If the ubiquitous "rich-get-richer" (RGR) phenomenon kicks in, we might end up with a couple of instances having all of the users. But did it kick in? Here is the relationship between instance's size and growth. There is a little bit of RGR, but some of the rapidly-growing instances are small. Decentralization also in how the network grows.