Recently, details of a party were posted on Twitter, resulting in 150 drunk youths gatecrashing it and ruining the house in which it was held.
This event is reminiscent of the huge party that made Aussie party boy Corey Delaney world famous. That particular shindig was promoted on MySpace.
These events give some idea of the massive reach of these sites. They also doesn't reflect too well on the kind of people who use them.