The concept is radical in its simplicity. An email or text is circulated among friends, friends of friends, or strangers. Participants flock to a designated public place, where they spontaneously engage in something out of the ordinary and, in doing so, astound onlookers: A Michael Jackson dance tribute, a lightsaber battle, a pillow fight. It’s systematized, attention-grabbing, and over in minutes.

In the decade following the first Manhattan flash mobs in 2003, these random acts of weirdness seemed a fun way to engage with the internet and mobile technology at a point when their value and purpose were still largely undefined. 

What a difference 20 years makes. In the age of social media, we accept as a given that virtual communication can lead to all manner of real-world action. T...