These things have popped up in the news a couple of times in the last couple of months, when large numbers of people, unified by an electronic summons / twitter / whatever, all descend on a single location to do something artsy. then leave to see if they made the news.
The government has noticed, and has a contract out to develop software to create a new kind of "flash mob" that may only exist on a flash drive somewhere. The thinking is that by creating realistic interactions between virtual people, the real ones can be led in the right direction.
I can see this working. 4 or 5 "alpha" twitterers put out messages endorsing some government policy, and the "beta" personalities fall into step behind, and the easily let real people, whom we'll call "twits", join the groupthink.
In a less benign environment, the "alphas" might call for a protest against the government at a specific time and place, the "betas" respond enthusiastically, and the police are waiting to pick up the "twits". Of course this can backfire when 250,000 "twits" show up with the local equivalent of ball bats.
Next killer app: Software to identify the virtual people.