The government is increasingly monitoring Facebook, Twitter and other social networking sites for tax delinquents, copyright infringers and political protesters. A public interest group has filed a lawsuit to learn more about this monitoring, in the hope of starting a national discussion and modifying privacy laws as necessary for the online era.

Law enforcement is not saying a lot about its social surveillance, but examples keep coming to light. The Wall Street Journal reported this summer that state revenue agents have been searching for tax scofflaws by mining information on MySpace and Facebook. In October, the F.B.I. searched the New York home of a man suspected of helping coordinate protests at the Group of 20 meeting in Pittsburgh by sending out messages over Twitter.

In some cases, the government appears to be engaged in deception. The Boston Globe recently quoted a Massachusetts district attorney as saying that some police officers were going undercover on Facebook as part of their investigations.

Wired magazine reported last month that In-Q-Tel, an investment arm of the Central Intelligence Agency, has put money into Visible Technologies, a software company that crawls across blogs, online forums, and open networks like Twitter and YouTube to monitor what is being said.