Did the youthful rioters who roamed the streets of London, Manchester and other British cities expect to see their photos scrutinized by angry Internet users, keen to identify the miscreants? In the immediate aftermath of the riots, many cyber-vigilantes turned to Facebook, Flickr and other social networking sites to study pictures of the violence. Some computer-savvy members even volunteered to automate the process by using software to compare rioters' faces with faces pictured elsewhere on the Internet.

The...