I'm personally not at all attached to the 'mastodon.social' domain (i'm self-hosting my own account on a different instance), but I am somewhat surprised at this perspective on TLDs.
Most users of the internet have been exposed to TLDs other than ".com". For example, wikipedia is at a .org TLD, US government sites are at .gov domains, university websites are at .edu domains. Most non-US users will frequently interact with their country's (and neighboring countries') ccTLDs, like .de, .uk, .in, ... I find it surprising to assume that users of social networks who have already understood abstract concepts like "like vs retweet" or "like vs share" would find it difficult to understand the difference between .com and .social.
Also in a sense, it is more accurate for Mastodon to be at a .social TLD instead of a .com since Mastodon is a Patreon-supported FOSS project, and isn't a commercial entity like twitter.com or facebook.com. But yeah, I know that .com doesn't really mean "commercial" anymore, and is more of a general-purpose TLD now.
Mastodon has a number of issues that could stifle broader adoption, but I can't convince myself that the TLD is really relevant here. Most users will just be linked to Mastodon from other sites, or find it from a web search. Once its in their web history, web browsers will just autocomplete the site name in the address bar. And isn't the domain squatting and exorbitant pricing on ".com" the main reasons why the new TLDs have been released anyway?