There are definite patterns around this, based on where you are, gender, what kind of bike you are riding, your clothing. In cold, early season, or in the dark, everyone waves. When I commute, other commuters wave, even though I am on a road bike/dressed in lycra, but I have a pannier. I notice more groups of men, out for training rides, are saying hi to me, or "have a good ride/nice day." Single male riders, not so much. I find women say hi/wave less.
Sometimes I just nod my head, but I do acknowledge. There are so many riders around here, especially in the places where I go for shorter rides, that I just don't wave to everyone. Of course, for me, this runs the risk I don't say hi to someone I know! DH and I rode right by She Fly a couple of weeks ago.