Increasingly, Facebook (Nasdaq:FB) is refusing to turn over user information that defense lawyers say they need to exonerate their clients. And to justify its position, the social networking giant is hiding behind a federal communications law first enacted when Mark Zuckerberg was 2.

Last week, attorneys for Facebook appeared at Multnomah County Circuit Court in Portland, Ore., to fight an attorney’s request to turn over a user’s information. The attorney wants access to a Facebook conversation between two friends of 17-year-old Parrish Bennette Jr., who is accused of fatally shooting 14-year-old Yashanee Vaughn and dumping her body in a local park. Bennette’s public defender, Thaddeus Betz, said he believes the conversation will help his client’s case, according to the Oregonian’s Aimee Green, who reported the story on Saturday.

However, Facebook is refusing to turn over the information, citing the Stored Communications Act, a 1986 federal law that addresses compelled disclosure of electronic communications and provides criminal penalties for those who intentionally access without authorization.

Betz subpoenaed Facebook for the information, but when the company did not comply, the judge assigned to the case ordered two Facebook attorneys to appear in court. The attorneys argued that the Stored Communications Act trumped the defendant’s constitutional right to gather evidence that might prove him innocent.

According to the Oregonian, one attorney told the court that “Facebook gets countless orders and subpoenas like this from courts throughout the country.”

Social networking sites such as Facebook and Twitter routinely comply with requests by law enforcement to turn user data that could facilitate the prosecuting of crimes. This past summer, for instance, Twitter initially denied a request by the NYPD to turn over account information for a user who was tweeting threats against a Broadway theater, but the site agreed to cooperate after it was subpoenaed. In all, Twitter reports that it complies with 75 percent of requests made by law enforcement in the U.S. without a subpoena.

Defense attorneys say that puts them at an unfair disadvantage, as prosecutors and law enforcement officials can gain access to Facebook posts, tweets, emails, Google search histories and other electronic communications that defense attorneys can’t. Moreover, they say, the federal law is perplexingly arbitrary. Defense attorneys are barred from obtaining electronic communications, despite the fact that they can legally obtain non-electronic records such as medical histories, bank statements and cell phone records.

Not all courts interpret the Stored Communications Act in the same way. In October, the South Carolina Supreme Court ruled that accessing someone’s email did not violate the act. The court argued that email accounts stored in “cloud” servers by companies such as Yahoo (Nasdaq:YHOO) and Google (Nasdaq:GOOG) do not meet the definition of stored communication as written in the 26-year-old law. The decision marked a split from a 2004 ruling stating that email that was downloaded and saved to a user’s computer did fit the definition of stored communication. Critics of the Stored Communications Act see this split as evidence that the act needs to be reevaluated by legislators.

As far as Facebook goes, it may not be off the hook yet. Bennette’s lawyer may still submit a second subpoena, according to the Oregonian. If Facebook fails to comply, it could be held in contempt of court, in which case, under Oregon law, it could face a fine of up to 1 percent of its annual gross income.