My problems with Mastodon aren't that it's not growing much. It's the fact the software itself is a cancer. It's easily the most greedy software running on my server both in terms of RAM and storage, and that includes when I run a Minecraft server.
Mastodon requires 3 separate processes, and one, sidekiq, leaks memory like a cheap diaper leaks piss. It slowly consumes storage space because there's no mechanism built in to cull cached avatars and header images of every single person who sees you on Mastodon. To solve these I wrote two cron jobs: one to restart the sidekiq process every 12 hours, and another a hackish attempt to delete unseen month old images. This shouldn't be needed.
The social network of the future: No ads, no corporate surveillance, ethical design, and decentralization! Own your data with Mastodon!