U.S. courts have a structural bias against “guilty” verdicts, but when it comes to Facebook data the situation is reversed: Social media activity is more readily used to convict you in a court of law than to defend you.

That’s because prosecutors generally have an easier time than defense attorneys getting private information out of Facebook and other social networks, as highlighted in an ongoing Portland murder case. In that case, the defense attorney has evidence of a Facebook conversation in which a key witness reportedly tells a friend he was pressured by police into falsely incriminating the defendant.

Facebook rebuffed the defense attorney’s subpoena seeking access to the conversation, citing the federal Stored Communications Act, which protects the privacy of electronic communications like e-mail – but which carves out an exemption for law enforcement, thus assisting prosecutors. “It’s so one-sided … they cooperate 110 percent anytime someone in the government asks for information,” one Oregon attorney told the Portland Oregonian, citing a separate case in which Facebook withheld conversations that could have disproved a rape charge, but turned over the same conversations when the prosecution demanded them.

Other defense attorneys voice similar complaints, and the judge in the murder case went so far as to call Facebook “flippant” and “frustrating” in its handling of the defense’s subpoenas. Facebook, for its part, has said it is inundated with judicial requests and tries to handle them uniformly within the confines of the law.

The trouble, it would seem, is that the law itself is not so uniform. As more and more communication shifts onto social networks like Facebook, the pro-prosecution bias of the Stored Communications Act is going to look less like a peculiar legislative oversight and more like a frightening erosion of the right to a fair trial. And if Facebook and its competitors want people to share more freely online, they should use their lobbying resources to fix that particular law.