This is 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."
Link to Original Source