The nude photo investigation that rocked the Marine Corps this week has shed light on Facebook's secret groups, a private and otherwise dark corner of the social network used by 1.86 billion people across the world.
At the center of it all is a secret Facebook group called "Marines United," where members allegedly shared nude photos of service women, along with lewd and lascivious comments.
While secret groups are, of course, held to the same standards as everything else on Facebook, it is obviously more difficult to know what possibly surreptitious activity is happening if the group is — well, secret.
Marines United, which was said to have tens of thousands of members and open to male service members and veterans, included "revenge porn, creepy stalker-like photos taken of girls in public, talk about rape, racist comments and just straight BS," according to a Reddit member who said he was added to the group and appalled at what he found.
It's unknown how many secret groups exist on Facebook; however, they tend to run the gamut from bachelorette party planning and local Mom groups to safe spaces for people tackling a shared challenge, such as depression or addiction.
Perhaps one of the most famous secret groups, Pantsuit Nation — a space for Hillary Clinton supporters that materialized during the presidential campaign — has 3.9 million members.
What Is a Secret Group?
A secret group is different than an old school or closed group, since members have to be added by other members, adding an extra layer of vetting to get in the front door.
While these groups have administrators, policing all of the content may present a challenge, according to Jen Golbeck, a professor at the University of Maryland's College of Information Studies.
"This can't all be [blamed] on Facebook. They can't possibly, accurately keep track of every bad thing that is happening," Golbeck told NBC News.
In the case of the private photos of service women, many of the photos were reportedly shared via a link in the group to a cloud drive.
"There isn't an automated way to catch that, so you have to have people looking for that. You have to have somebody in the group flag it," she said.
In a statement to NBC News, a Facebook representative said, "We want to have an open and safe environment on Facebook. We do not allow harassment and remove content that appears to purposefully target private individuals with the intention of degrading or shaming them. We also remove content that threatens or promotes sexual violence or exploitation."
Who Should Report Offending Content?
Part of the onus also falls on Facebook's users, who are also encouraged to report offending content for review. Facebook also suggests "reaching out to law enforcement directly if there is a situation in which the authorities can help," the statement said.
Asking users to flag offending content may be partially effective, but Golbeck said there's a real risk of becoming a social pariah in these closed groups if you tip off Facebook to objectionable posts.
"There is this risk that even if what is going in on the group is bad, you're going to tick off a bunch of people in this exclusive club you are a part of," she said. "The social aspects of that might change."
In the case of Marines United, all it took was one person to blow the cover of what was happening in the secret group.
The War Horse, a nonprofit news organization run by Marine veteran Thomas Brennan, first reported the activity in the secret group to the Marine Corps on January 30. The investigation was first reported by the Center for Investigative Reporting.
Gen. Robert Neller declined to comment specifically about an ongoing investigation, but released a statement that read: "For anyone to target one of our Marines, online or otherwise, in an inappropriate manner, is distasteful and shows an absence of respect... I expect every Marine to demonstrate the highest integrity and loyalty to fellow Marines at all times, on duty, off-duty, and online."