Be careful who you “friend” online.

U.S. law enforcement personnel have begun to use social networking sites — Facebook, LinkedIn and Twitter, among others — to gather information for investigations, according to an internal Justice Department document obtained by the Electronic Frontier Foundation through a Freedom of Information Act lawsuit.

In December, the Electronic Frontier Foundation and students from the law school at the University of California at Berkeley filed a suit in California federal court seeking information from six federal agencies on their use of social networking sites for investigations and data gathering.

In response to the suit, the IRS and Justice Department both released documents to the civil liberties group. The U.S. Marshals Service and the Bureau of Alcohol, Tobacco, Firearms and Explosives both said they had no documents on the use of social networking by agents.

The foundation posted the Justice Department document, titled “Obtaining and Using Evidence From Social Networking Sites,” on its Web site Tuesday.

The document, compiled by the DOJ’s Computer Crime and Intellectual Property Section, offers an introduction to several major social networking sites, including Facebook, MySpace and Twitter. It also makes note of how responsive certain sites are to law enforcement requests. Facebook, for example, is often responsive to emergency requests, the document states, while Twitter produces data only  in response to the legal process.

The presentation also covers the potential legal and practical implications of using social networking sites for undercover operations. The document notes how the case U.S. v. Drew — in which a Missouri woman was charged with violating MySpace’s terms of service for creating a fake profile — could affect undercover operations. “If agents violate terms of service, is that ‘otherwise illegal activity?’” the document asks.

Critics have noted that the IRS and DOJ documents make little mention of oversight of investigators’ use of social networking sites.

“It doesn’t really discuss any mechanisms for accountability or ensuring that government agents use those tools responsibly,” Marcia Hoffman, an attorney with the Electronic Frontier Foundation, told The Associated Press.

D.C. lawyer Marc Zwillinger, a former trial attorney in the Justice Department’s Computer Crime and Intellectual Property Section, also said there should be limits on undercover use, according to the AP.

“This new situation presents a need for careful oversight so that law enforcement does not use social networking to intrude on some of our most personal relationships,” said Zwillinger.