I usually sound the bell for everybody I'm overtaking on a bike path (at a distance and in a way that won't startle them), whether they are walking, skating, or cycling. I didn't use to (didn't even have a bell for years), but I find even other cyclists are just as unpredictable as walkers. And both can startle easily and veer into your path or right into you. I don't take any chances, having had way too many close calls. It just takes one moment of inattention or looking at something along the way for a cyclist to veer into your path as you're passing (and sometimes there's just nowhere else to go because of a railing, trees or whatever). Then there are the yahoo cyclists who pass me at high speed *while* I'm passing someone else on a relatively narrow path! That's really smart. Sometimes I think the roads are safer.

By the way, sometimes I'm a pedestrian on multi-use bike paths. When some idiot is suddenly on your rear end at high speed and then passing in the one foot wide space between you and bike or pedestrians going the other way, it can be more than a little un-nerving.