As others have said, Mastodon.social is fine, you have not screwed up. Are there some downsides to Mastodon.social? Sure, but that goes with any server, or any platform or any choice you make in life (haha). Some folks on Mastodon (or the Fediverse at large) deride Mastodon for A. being “too big”/centralized, B. not liking their moderation practices, C. their association with efforts such as Meta’s Threads federating, and/or D. certain accounts they have that folks may not like elsewhere. Whatever. Mastodon.social is FREE to join, is obviously widely federated - which helps with reach and search, and since it is the flagship instance, has decent moderation, good support, bleeding-edge features, etc..

Also, as others have said, you can migrate your followers elsewhere later if you decide you want a smaller instance, or don’t like mastodon.social for some reason, etc.. The only caveat is right now you can’t also migrate your posts, but I personally don’t see that as being a big deal. If you care about things you post over there, you can download a post archive, or maybe just archive your good stuff on a personal blog or something.

I have an account on Mastodon.social, an account on a secondary (much smaller instance) and a self-hosted instance (yeah I know, I have a problem). There are pros and cons to each experience. What’s important is that you made it out (of the centralized platforms) and are building a presence on the open, federated, social web. Congrats! You can also ask for help there using hashtags like (#fedihelp, #askfedi) and you’ll be sure to get lots of responses.