Here in the States we tend to give other riders a little wave instead of just a nod. Mostly it's become a sort of stylized flick downward off the left side of the handlebar...often just a couple of fingers. Still, it's a nice "we're all in this together" gesture of acknowledgement. As for me, I'll give anyone on 2 wheels (or 3) a proper wave: scooters, mopeds, even bicycles on a country road. I ain't snooty.
I do the same. It's rare a motorcyclist doesn't wave back - but when they don't it's always a Harley rider. Jes sayin'. We don't have hardly any moped or scooter riders, they're not all that useful in my somewhat rural environment. There's plenty of bicyclists in the summer, sometimes I wave to them, too - they never wave back, probably don't know what to think of me. Usually, though, the bicyclists are scoffing the traffic laws, which bugs me because they want to be treated like real vehicles on the roads but they don't want to follow the vehicular rules when it doesn't suit them - I don't wave at those guys and gals.
Once I waited at a 4-way stop while eight or ten cyclists, all single file with three or four seconds between them, blew right through the stop sign on my left, ignoring my right-of-way and just waving as they went by. I finally got my chance to turn and threaded by them one by one, berating each for ignoring the stop sign. Feels good to be a cranky old man sometimes...
If you get out into the remoter stretches of the west, like West Texas, where one might not see another car for an hour or more, even car drivers give each other a wave.
All drivers wave at each other on the Maine coastal islands. But friends get a proper wave and eye contact - foes (and there's plenty of 'em out in these northern redneck communities) get a raised finger or two (no, not
that finger!), but at the same time the driver turns his or her head and looks away, effectively waving and snubbing simultaneously. They're great multi-taskers, those islanders...