I am one of those cyclists who pass peds and others on the trails of Lincoln Park and elsewhere in our city.
I save the 'on your left's for other bikers, when passing someone moving faster than foot-speed I'll add 'jogger, boarder, mom or dad (if pushing a stroller)' etc to the verbal warning. Always try to do it a second time if it appears they did not hear me.
When coming up on a group where they are likely to scatter like chickens, from even farther back I'll yell something to the effect of "good morning", or "I'd like to pass please". Something that will get the attention of one or more that will (should) result in a safe passing.
If any of the above don't work after two soundings because they're not paying attention or have wires hanging from their ears, then I'll pass anyway, aware of the space I'm leaving between us. Not menacingly, yet guilt-free as I tried doing my part.
Of course there are those who are legitimately hard-of-hearing/deaf, and I'd like to think most of them are hyper-sensitive to situations like that. However, we know humans right?!!!
In the Marine Corps (and I'm sure in other branches of the service and LEO training) we learned about 'situational awareness', KNOW WHAT'S GOING ON AROUND YOU!!!.
It may be our (bikers, faster persons) responsibility to pass you safely, but what about your responsibility of keeping yourself safe?
Heck it's really easy to look behind you every couple of minutes to see if potential trouble is headed your way.
What if you get one of the a-holes on a saddle who don't give a poo-poo and fly by, smacking an elbow with the handle bar causing the two scoops of ice cream to fly off their cone and into the group, with the strawberry one landing in Grandma's purse?! (I actually witnessed this happen at Gas Works park a few years ago)
In utopia bike paths are ONLY for bikes, here on earth we're all in the mixing pot together, lets make something delicious out of it.