Two datapoints for your consideration:
Digg Recovers a Stolen Xbox. A guy in Philadelphia gets back from SXSW to find that his house has been burgled and his Xbox stolen. He gets a new Xbox and receives a message via Xbox Live asking for a ransom to get his old console back. The message comes from an actual valid gamertag. The guy calls the police, but they don’t care. So he blogs about it, publishing the offending gamertag, and the blog entry gets Dugg. Readers trace the gamertag and relentlessly harass the Xbox-napper till he gives up and returns the hardware.
Crazed Craigslisters Steal Everything a Guy Owns. Somebody posted a Craigslist ad saying that such-and-such a house had been abandoned and that everything in it was free for the taking. The ad was a hoax, but people actually turned up at the house in question, raided it and stripped it. Needless to say the actual owner was not best pleased when he got home.
There you go, two parables of the power of the Net to effect spontaneous collective action, for good and for ill. You almost wonder why this kind of thing doesn’t go on more often, and why people don’t push it further. People love to be cyber-vigilantes. What couldn’t you get them to do, with a sufficiently plausible come-on? And this kind of thing is only going to accelerate and get scarier once the mobile Net becomes more prevalent. Imagine you’re on the subway (or whatever), and the other people in the car get a message on their iPhones (or whatever) that you’re a known felon, or a racist, or a fan of masochistic sex. The mind boggles.