You don't need to join mastodon.social or any other big servers on here.
Big servers do not mean big audiences on the Fediverse. Because federated servers talk to each other seamlessly, the number of people you can interact with is similar whether you're on a large or small server. Some people even run their own single user servers where they are literally the only member, yet they are able to interact with just as many people as a user on a megaserver like mastodon.social.
Mastodon and the wider Fediverse were designed so that their users could be spread out across many different servers, specifically so that the network couldn't be taken over by malevolent billionaires. By choosing a smaller server, you help keep the network safe from any takeovers. Smaller servers also tend to be better moderated and have better tech support because they have a lot more staff per user.