I've actually had a lot of success explaining this all like email; where you and I can create our accounts on different "email services" (instances) yet still be be able to communicate. Using something so ingrained in every day life makes it much more approachable, imo.

Once that concept is understood, simply saying that the fediverse, aka everyone who uses this technology, has alternatives to most of the major social media sites is not beyond most people. I mean, we're all here, right?