I almost never say "passing on your left" because at least half the time the person will move to the left.. I just say "PASSING" and then wait and see what happens. Some people jump as if they expected no one else to be on the MUP. Some move over and wave, I make sure to say thank you as I pass. If there's two or more I wait for the 'people pin ball' to stop. You get the idea.
Joggers with headphones usually don't respond, so I just pass them.