I was thinking about this thread last week...there is a bridge that I and other cyclist use the sidewalk to cross the river. I was coincidentally following another cyclist who was attempting to pass a group of walkers. She did the "on your left", and whether she meant it or not, it did sound a bit rude. One of the walkers did an exaggerated motion of moving out of the way. When I went through I did thank them and got a you're welcome back. Personally, I like to say "hello" or "excuse me" when needed for passing and thank them afterwards.