Web services copied this from the Unix world, where it was popularized by the vi text editor, nethack, and a bunch of other programs. If you used a Unixy text mode email program like elm, mutt, or pine, then you had j/k for up/down.
Did you know that "j" and "k" are often used as up/down or previous/next keys?
Yes, I use j/k for up down in some application(s) often.
I know this, though it's not something I use much currently.
Seems familiar... you just reminded me, but I think I knew.
Doesn't sound familiar.