I give the finger twitch or head nod to all the cyclists that are coming the other way and get very few returns, if I do it is usually from the older cyclists.
It doesn't bother me, but I think they had to have seen it, otherwise they are not very aware of what is going on around them, which is not very safe.
It seems to be very common for people on motorcycles to wave at others as they pass. It's like being in a special club...