@youronlyone I'd say you can just roughly describe what it is people do on, say, Mastodon, mention one or two core features, and then you can use some comparisons like "it works kinda like Twitter" and mby add some differences between the 2.
I think that focusing on what the platform is and what its strengths and features are upfront, instead of e.g. introducing it as a Twitter clone and THEN talking about its features, establishes a different set of expectations for the newcomers.