I don’t know how many people are familiar with popular websites in Japan, but 2ch is the most widely used online forum. The interface is absolute crap, but that’s par for the course when it comes to Japanese websites. Check out the Wikipedia article for more info. It’s pretty interesting, especially the story about Densha Otoko.
On the first of this month a 23 year old vagrant used his cell phone to post a message on 2ch saying that he was going to kill an elementary schoolgirl in Chiba Prefecture on Feb 15 at 3pm. As a result, over 15,000 community volunteers and parents in Chiba were mobilized to ensure the safety of children going to and from the nearly 1,300 schools in the prefecture. When the police tracked down the person who posted the threats, he said he wrote the messages because he thought the replies were funny. The authorities disagreed.
Another link, so you can see what a typical 2ch thread looks like.
If you want to see how far and wide his threats reverberated through the net, Google this: 千葉の女子小学生を２月１５日１５時に殺しちゃいます