Don’t.

Just say: “this is like Reddit(/Twitter/…), but independent."

They will figure the rest if they have or want to.

As others have said, just don’t bring it up unless specifically asked.

Most users won’t care, and you might easily overwhelm them with data.

It is far better to introduce them to Lemmy/Piefed/mbin/kbin/Pixelfed/etc as a specific service, and answer further questions as they come up.