Mastodon-the-software is used by far by the most people on fedi. The biggest instance, mastodon.social, is home to over 200.000 active accounts as of this writing. This is roughly 1/10th of the whole Fediverse, on a single instance. Worse, Mastodon-the-software is often identified as the whole social network, obscuring the fact that Fediverse is a much broader system comprised of a much more diverse software.