Anthony Elonis was convicted of violating a federal law that bars making interstate threats to harm people after posting the threats about his wife and area children.
In a narrow 7-2 ruling issued on Monday, the Supreme Court ruled that that the standard used to convict Elonis wasn't sufficient.
In a majority opinion, Chief Justice John Roberts said that prosecutors needed more proof to show that Elonis had intended to act on his online threats.
The government argued that the threats only needed to reach a standard where a "reasonable person" would interpret the posts as threats, but the majority opinion suggested that Elonis' own intentions were also important.
"Federal criminal liability does not turn solely on the results of an act without considering the defendant's mental state," the court ruled, sending the case back down for further review with the stricter standard.
Elonis' string of damning Facebook rants came after his wife of seven years left him and took their two kids with her in May 2010, according to the petition asking the Supreme Court to hear the case.
Elonis made questionable Facebook posts about his former employer (an amusement park that fired him), followed by posts about his estranged wife, and then a post in which he appeared to threaten elementary school kids. That last post prompted the FBI to start monitoring him, which in turn inspired a derogatory post from Elonis about the "Little Agent Lady" who came to his door.
The court dodged a chance to potentially make a wider argument about online free speech, opting instead to issue a narrow ruling about Elonis's case in particular.
According to WFMZ, Elonis is currently back in jail after violating the terms of his release from prison. In April, Elonis was arrested for simple assault and domestic violence.
Elonis argued that his posts - many of which were in rap form - weren't actually meant to threaten their subjects, and that he had a First Amendment right to make these statements. He has support from a number of free speech groups, including the ACLU.
"The inherently impersonal nature of online communication makes such messages inherently susceptible to misinterpretation," his lawyers wrote in his petition.
The subjects of Elonis' Facebook communications obviously interpreted his posts as threats, and it's not hard to see why.
In one Facebook comment, Elonis suggested that his son "dress up as matricide" for Halloween. His wife obtained a Protection from Abuse order, prompting him to post a longer rant about her that was basically a word-for-word adaption of sketch by a comedy group they'd seen together, according to his petition.
"Did you know that it's illegal for me to say I want to kill my wife?" he wrote. "It's illegal. It's indirect criminal contempt ... I also found out it's incredibly illegal, extremely illegal, to go on Facebook and say something like the best place to fire a mortar launcher at her house would be the cornfield behind it ..."
His post about the FBI agent is more graphic. In it, he noted that it "it took all the strength I had not to turn the b**** ghost. Pull my knife, flick my wrist, and slit her throat."
Like a lot of rap lyrics, Elonis' have a violent undertone. His words got him 44 months in prison and three years of supervised release.