The "Low Five" has been around since I started riding on the street back in the 80's, or longer.
Sometimes i see the Hardley guys give the wave, and then choke in horror when they get close enough to realize they aren't waving to another Hardley.
They almost never return the wave if I am on my sport bike....
Dudes on scooters don't wave at all as a rule. If they wave at me, and then try to catch up to me I either loose 'em with some throttle, or hit them with the bear spray. Sometimes I spray 'em even if there was no waving at all, particularly if they are wearing sandals or any artical of clothing that is plaid......Its highly effective.