Just send them the app url and let them know you do private chats on a private messenger. Show them all the fun stuff like stickers, GIFS, groups, stories, etc. Save the delete messages stuff for later.
What "app" are you using (for Lemmy) or just the browser? Memmy's incredible
There's nothing elitist or unreasonable about this. Lots of friendly people to help them if they need some direction or getting up to speed.
Just let them know you don't want Facebook having anything more on you than they already do. Let them know it has everything people want in messaging apps like stickers (whatever the heck those are?!), GIFS, video and phone calls, groups, etc.
Who cares now? Lemmy is the future and it just as well that Reddit closes itself off. I'm over it :/