All you really need to do is find someone who is alone on an empty street and step close enough to be within the 45 metre range necessary to share photos with complete strangers—without being spotted. Then post a few comments referencing your victim's location on his or her photos.
Try to keep your laughter quiet as you watch your poor victim whip his or her head around in an attempt to figure out where the creepy stranger—yes, that's you—is hiding.
It'll feel like you're ten and playing ding-dong-ditch or something similar all over again. [Twitter]