NEW YORK (CNNMoney.com) -- You might be surprised to learn who's following you on Twitter, or who your Facebook friends really are.
As the popularity of social networking spreads, law enforcement agencies are tapping into these sites to nab criminals, tax evaders and other wrongdoers, and gather evidence to support their cases.
"People don't think [authorities] are going to go that far, but little do they know, they are going this far," said David Seltzer, a criminal and cyber crime defense lawyer.
In several of Seltzer's cases, law enforcement agents created a false profile on MySpace and "friended" a suspect or a suspect's friends in an attempt to retrieve information they needed for an investigation.
"I always tell my clients, if you have any social media pages, take them down, because as soon as something happens, agencies will start Googling your name," Seltzer said.
Last month, digital rights advocacy group the Electronic Frontier Foundation (EFF) obtained internal documents from the Justice Department and the IRS showing the ways in which social networking is used during investigations.
For example, an internal Justice Department presentation explained to employees that using social networking in criminal cases can reveal a suspect's communications or whereabouts, establish motives and personal relationships and prove or disprove alibis.
As long as the information is public on sites like Facebook, LinkedIn and MySpace, it's fair game for law enforcement. The Department of Justice can also take legal measures to retrieve private data from the site owners, according to the presentation.
"We will continue to use publicly available information individuals post online about their illegal activities or false statements to law enforcement officials in our investigations," a spokeswoman for the Department of Justice said. In addition to accessing public information through social media, the Justice Department document explains how going undercover online allows agents to communicate with suspects and targets, gain access to private information and map social relationships and networks.
But in order to do so, they need cooperation from the sites.
A spokeswoman for MySpace said the site has created a law enforcement guide and has developed a 24/7 hotline and e-mail account to assist law enforcement investigations. The company also provides training for cyber crime units on how to investigate and prosecute cyber criminals using MySpace.
The DOJ said in its presentation that MySpace often has public profiles but that it requires a search warrant to view private messages less than 181 days old.
"Ultimately everything we do revolves around two things," a MySpace spokeswoman said. "Making sure law enforcement gets the information they need in a way that complies with all laws in order to be admissible in court, and protecting the privacy of users from unauthorized exposure."
Facebook rarely allows for emergency exemptions from privacy laws and will fight requests it believes violate the law, according to a spokesman for the company.
"One hypothetical is a kidnapped child where every minute counts," he said. "It is in this type of instance where we have verified an emergency that we feel a responsibility to quickly share information that could save someone's life."
Even in this example, however, the spokesman said Facebook would share the minimum amount of information, such as whether a user has logged in to his or her account.
Twitter is less cooperative. While most content on Twitter is public and private messages are kept until users delete them, the site doesn't require contact information, so users are tough to identify. And the site will only turn over information in response to legal process, according to the DOJ presentation. The IRS also uses social media sites like Facebook, Twitter, LinkedIn, YouTube and even Google Street View to investigate taxpayers.
While the Department of Justice acknowledges going undercover online, the IRS prohibits employees from misrepresenting their identities to obtain information on social media sites.
But IRS agents are allowed to use information they find about an individual taxpayer or business if it is made publicly available on a social networking site.
"It's presumably just a really cheap way to see what someone's house looks like," said Shane Witnov, a student at the Samuelson Law, Technology & Public Policy Clinic, who worked with EFF to obtain this information. "If someone says their house is worth $100,000 and the IRS looks at it on Google Street View and it's a mansion, they could probably question that claim."The Electronic Frontier Foundation questions the extent to which federal agencies should be able to use social media without crossing the line of legality and privacy invasion.
"The documents basically confirmed what we knew, that social networks are being used to collect information for investigations," said Witnov. "But we're still trying to find out the scope of their use and what sort of oversight is in place to limit it, since it could be a potential invasion of privacy."
Witnov says that in some cases, authorities may be overstepping their boundaries, especially when creating false profiles and online identities to collect information.
Land O'Lakes CEO Beth Ford charts her career path, from her first job to becoming the first openly gay CEO at a Fortune 500 company in an interview with CNN's Boss Files. More
Honda and General Motors are creating a new generation of fully autonomous vehicles. More
In 1998, Ntsiki Biyela won a scholarship to study wine making. Now she's about to launch her own brand. More
Whether you hedge inflation or look for a return that outpaces inflation, here's how to prepare. More