When I owned a Triumph TR-7 and passed another TR-7 or -8 during the day, I would make my lights flip up & down as a "wave".
Of course with a Triumph, that was both a sign of friendliness and showing off because the headlight motor on most of those burnt out after about four years.

I am generally a friendly kind of person so tend to do the finger wave or head nod. But not always. And I usually don't notice or care if the other cyclist returns the gesture.