If they are on two wheels, I wave if I can. I live in a small town and there are a lot of kids that ride mopeds. Hell I even wave at them. They are just kids on scoots that will be future Harley riders!
I do think it is sorta funny when I am riding down the highway (2 lanes each direction) and the guy on the bike going the other direction 200' away is more concerned about waving at me than watching the road. But whatev...if I notice them I give a wave.
I think waving is gay. Never did get it. I own a car and don't wave to every car that I go past! It is annoying to me.
It's funny you say that, because Corvette owners used to wave at each other starting back in the '50s. I imagine it was because they were probably seldom seen then, unlike Corvettes or motorcycles nowadays.