I’d probably just choose a server and send it to them. Then, I’d explain it like this:

Kbin is like Reddit, but (assholes|fuccbois|corporate interests) can’t control it like they can control Reddit. It’s a bit rough around the edges, but it’s getting better every day. You can try it out by going to this link and signing up: <insert link>
If you want to know more about why it can’t be controlled and ruined, look up the “fediverse.” None of that matters if you just want to have a Reddit alternative though, you’ll figure it out as you go along.

EDIT: the instance you send them to should be a general purpose one, btw.