I normally wave at bikes, trikes, and scooters. And little kids. And anyone I know. And anybody who looks like they might appreciate it. It's my way of saying something to the effect of "It's a great day to be on a bike!". If circumstances (traffic, road conditions, speed, intersections, etc) are a bit strained, I don't wave because the other rider doesn't need the distraction, and I prefer to stay focused as well.
One of my best friends, who is my most frequent riding partner, is one of the friendliest guys I've ever met, but he hardly ever waves. So I've learned that everybody waves or doesn't wave for their own reasons. Lots of us have an "Us" vs "Them" attitude about waving. Nothing wrong with that, IMHO, as long as we don't take it to seriously. Every time I think that (whatever type) of riders/bikes never wave, it seems like the next few I see wave to me first.
All in all, it seems to me that the farther out of town, and the hotter or colder or more miserable the weather, the more likely we are to wave at each other.
FWIW, the other day we (C14 & Tiger 955) saw a black Spyder with a rider all dressed in black with a mohawk on his helmet. His tags were an abbreviation of "Batmobile". We waved, he waved.