There's no real correlation I can find - the other user just needs to be online.
I used to think it was someone who had liked, viewed or replied to another post but as often as not the person hasn't even left the log in screen (as far as I can tell)
Maybe some activity from the OP is required, but I haven't seen it EVERYTIME.
I think we just seem to be swapped with the same active people because they are the ones online the most