Any criminals dumb enough to brag about their exploits on social networking sites have now been warned: the next Facebook "friend" who contacts you may be an FBI agent.

US federal law enforcement agents have been using social networking sites ‑ including Facebook, LinkedIn, MySpace and Twitter ‑ to search for evidence and witnesses in criminal cases, and in some instances, track suspects, according to a newly released justice department memo.

FBI agents have created fake personalities ‑ in apparent contravention of some of the sites' rules ‑ in order to befriend suspects and lure them into revealing clues or confessing, access private information and map social networks.

The new online efforts were revealed in a justice department document obtained by the Electronic Frontier Foundation, a San Francisco-based legal advocacy group. The document, a 33-page slideshow prepared by two justice department lawyers, was obtained in a lawsuit the group filed against the justice department, seeking information on its social network policies.

Law enforcement agencies have long used internet chatrooms to lure child pornography traffickers and suspected sex predators and with a warrant, can seize suspects and defendants' email records. But Facebook, MySpace and other social networking sites provide a wealth of additional information, in photographs, status updates and friend lists. In many cases, the information is publicly accessible.

In a section entitled "utility in criminal cases", the document says agents can scan suspects' profiles to establish motives, determine a person's location, and tap into personal communication, for instance through Facebook status updates.

Agents can examine photographs for guns, jewellery and other evidence of participation in robbery or burglary, and can compare information on Facebook status updates and Twitter feeds with suspects' alibis. Friend lists can yield witnesses or informants.

The document advises agents that Facebook is now used in private background checks. It indicates that Facebook often co-operates with emergency law enforcement requests.

In one section on working undercover on social networking sites, the document poses but does not answer the question: "If agents violate terms of service, is that 'otherwise illegal activity'?"

Facebook rules bar users from providing false information or creating an account for anyone other than yourself without permission, and says that users should "provide their real names".

A former US cyber-security prosecutor told the Associated Press that federal investigators working online should be able to go undercover as much as they do in the real world, but said rules need to be developed.

"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," Marc Zwillinger said.

In one case that highlights the use of social networking in law enforcement, a man wanted in Seattle on bank fraud charges fled and police lost track of him. The suspect's Facebook page was private but his friend list was public. Among Maxi Sopo's friends, prosecutors spotted a former justice department employee who did not know he was wanted. When Sopo posted messages on Facebook describing his easy new life in Mexico, his online friend provided information that enabled Mexican police to nab him in September.