Like CB HI, I tend to ride to the right side of the path anyways, so I have no problem with people passing unannounced a few feet away. When passing, I use my bell to warn the jagoffs who ride right down the middle, or when there are two or more people side-by-side. I've given up yelling on your left because so many people take that to mean "move to the left".

Aside: I had a jogger a few weeks ago yell "You're supposed to signal" at me after I passed him on the MUP. The funny thing was that that part of the trail was empty, I wasn't going particularly fast, and I gave him about 4 feet of clearance when I passed. I guess he randomly likes to jump several feet from side-to-side as he runs...