At a closing speed of 30-40 mph, it is difficult, if not impossible, to have a meaningful exchange, IMHO. A wave or finger salute is pretty much meaningless. I will yell out "watch it," "heads up," "leash" or some other short phrase to indicate unsafe behavior. Other than that, my bell is used frequently to indicate my presense and intentions. As I encounter hundreds of bike riders every day when the weather is good, concern with developing a community is not a priority. Safety is pretty much all I care about.