The people I follow on mastodon come from a wide variety of instances. While mastodon.social is the largest instance, most of the accounts I follow are elsewhere.

Granted, all the smaller instances are likely easier to DOS as they are small instances. But mastodon is actually decentralized. If any one instance goes down, everything else keeps working. Unlike Bluesky and ATProto which is more of a theoretical “could be” decentralized.