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.