Via smart mobs :

Flash Mob : spontaneous gathering of people (familair with each other OR NOT) in the same place. Like these inexplicable mobs in NY : “groups of pranksters who use email to agree upon a place and activity, then descend upon an unsuspecting proprietor of a bar or department store, to act out pre-arranged antics”

The term “flash mob” refers to “flash crowd”: “Larry Niven’s 1973 SF short story “Flash Crowd” predicted that one consequence of cheap teleportation would be huge crowds materializing almost instantly at the sites of interesting news stories.”