The biggest problem with Mastodon is that 3 instances comprise 50% of all traffic. That’s not that bad but that creates the conditions for the largest X instances to become Mastodon Inc., use VC money to fund advertising for their site and fund improvements the other instances don’t get, eventually defederate from the other instances, and finally enshittify. I think Mastodon is a bit more insulated from this than fully private companies, but it’s not invulnerable while that many users are on the biggest instances.