It's an absolutely massive instance that's a net negative for the Fediverse. It completely defeats the purpose of federation. The Mastodon devs used to drive people to smaller instances but decided they wanted to be /the/ instance. They made themselves a default in the app. At around the same time other instances started getting a crap ton of spam from them that ate up a bunch of moderator's time on smaller instances. The Fediverse only works in regards to moderation because there is less users per admin, but mastodon.social doesn't have that advantage. A bunch of people defederated from them as a result which was a good thing for the instances that did it. They failed pretty hard at communicating during this time as well.
Having one instance hold a large part of the network is bad for everybody involved. Defederating from monoliths is a healthy thing for networks to do. Building your own web beats any algorithm, can't do that if you're already federating with 99% of people.
The Fediverse is dominated by hackers, who by their nature are incompatible with existing systems such as capitalism.