People who don't use other social media apart from Lemmy... how do you explain this decision to the average Joe with accounts for all existing social media?

Not sure if you're distinguishing social media from messenging. I am, so as far as discussing social media:

I don't. Why would I? In Germany, at least for the 30+ population WhatsApp is the basic consensus for instant messaging (albeit there seems to be a trend towards instagram as a messenger - sharing instagram handles instead of phone numbers among younger people?). I'll have to accept that, or not communicate.

Anyways, I have a handful of people who are on Signal, a couple of people on Telegram, one on Threema, and I installed them all and use DM Me to pick the "best available" option if I need to start a conversation with someone and can't just reply to a notification.

As for other social media (lemmy), people either know it, or they don't care either way. Zero reason for discussions.