I don't bother waving. No offense.
It's either:
1) We're all commuting, and nobody really feels like going "Hello, sunshine!" when they're going to work;
2) It's some training loop, and they're in too much pain to wave, "Hello, sunshine!"
But, trading pulls can make for decent conversation to break the monotony, and waving to a cyclist right next to you is kinda silly (and risky).
3) There are too many others out riding, and I don't like riding one-handed for any length of time.