If I'm on the shared path I'll say hello to any other cyclists, joggers, dog walkers and the people just getting from A to B, I do it because I'm nice like that. If I'm on the road I'll give a wave, or a nod or a hello to the cyclists I see, sometimes all three!
It must be my gregarious nature I suppose, perhaps somebody should think about starting some sort of on line community for cyclists, a forum if you will, where we could have a chat about a shared interest, after all we don't usually have time to stop and chat to one another on the roads, but it's nice to know there are more of us out there and acknowledge each others existence.