Bill Wasik writes: Let’s start with the fundamental paradox: Our personal technology in the 21st century—our laptops and smartphones, our browsers and apps—does everything it can to keep us out of crowds.

Why pack into Target when Amazon can speed the essentials of life to your door? Why approach strangers at parties or bars when dating sites like OkCupid (to say nothing of hookup apps like Grindr) can more efficiently shuttle potential mates into your bed? Why sit in a cinema when you can stream? Why cram into arena seats when you can pay per view? We declare the obsolescence of “bricks and mortar,” but let’s be honest: What we usually want to avoid is the flesh and blood, the unpleasant waits and stares and sweat entailed in vying against other bodies in the same place, at the same time, in pursuit of the same resources.

And yet: On those rare occasions when we want to form a crowd, our tech can work a strange, dark magic. Consider this anonymous note, passed around among young residents of greater London on a Sunday in early August:

Everyone in edmonton enfield woodgreen everywhere in north link up at enfield town station 4 o clock sharp!!!!

Bring some bags, the note went on; bring cars and vans, and also hammers. Make sure no snitch boys get dis, it implored. Link up and cause havic, just rob everything. Police can’t stop it. This note, and variants on it, circulated on August 7, the day after a riot had broken out in the London district of Tottenham, protesting the police killing of a 29-year-old man in a botched arrest. So the recipients of this missive, many of them at least, were already primed for violence.

It helped, too, that the medium was BlackBerry Messenger, a private system in which “broadcasting” messages—sending them to one’s entire address book—can be done for free, with a single command. Unlike in the US, where BlackBerrys are seen as strictly a white-collar accessory, teens and twentysomethings in the UK have embraced the platform wholeheartedly, with 37 percent of 16- to 24-year-olds using the devices nationwide; the percentage is probably much higher in urban areas like London. From early on in the rioting, BBM messages were pinging around among the participants and their friends, who were using the service for everything from sharing photos to coordinating locations. Contemplating the corporate-grade security and mass communication of the platform, Mike Butcher, a prominent British blogger who serves as a digital adviser to the London mayor, wryly remarked that BBM had become the “thug’s Gutenberg press.”