A year ago this past Friday, Mark Zuckerberg published a lengthy post titled “Building a Global Community.” It offered a comprehensive statement from the Facebook CEO on how he planned to move the company away from its longtime mission of making the world “more open and connected” to instead create “the social infrastructure ... to build a global community.” He identified a number of challenges to realizing his mission, and ranking high among them was the political polarization of his user base.

“Social media is a short-form medium where resonant messages get amplified many times,” Zuckerberg wrote. “This rewards simplicity and discourages nuance. At its best, this focuses messages and exposes people to different ideas. At its worst, it oversimplifies important topics and pushes us towards extremes.”

By that standard, Robert Mueller’s indictment of a Russian troll farm last week showed social media at its worst. The special counsel filed conspiracy and fraud charges against 13 Russian nationals and three organizations as part of a widespread, multimillion-dollar effort to influence the US election. Adopting US identities and personas, they created a variety of groups and pages across Facebook, Instagram, Twitter, and YouTube for the express purpose of pushing Americans to extremes.

Facebook has estimated that 126 million users saw Russian disinformation on the platform during the 2016 campaign. The effects of that disinformation went beyond Likes, comments, and shares. Coordinating with unwitting Americans through social media platforms, Russians staged rallies and paid Americans to participate in them. In one case, they hired Americans to build a cage on a flatbed truck and dress up in a Hillary Clinton costume to promote the idea that she should be put in jail.

News of the indictments come at a time when Facebook is investing more in its products for groups. Last year, Facebook said 100 million people are in what the company calls “very meaningful” groups, or groups that are a primary part of the user’s social networking experience and extend to offline interactions. A parenting group might be very meaningful to a young family, for example. In his post last year, Zuckerberg said Facebook hoped to increase the number of people in very meaningful groups to 1 billion.

But what if those very meaningful groups are run by foreign actors working to make the country more polarized? It’s impossible to say how “meaningful” the groups Russia created were to its members, but the troll farms worked to create pages around subjects that generate the maximum level of emotion. Often, they were tied to identity. For immigration matters, there was a page called “Secured Borders.” For Black Lives Matter, there was “Blacktivist.” For religion, there were “United Muslims of America” and “Army of Jesus.” By 2016, those pages collectively had hundreds of thousands of American followers.

Russians spent thousands of dollars a month promoting those groups on Facebook and other sites, according to the indictment. They meticulously tracked the growth of their audience, creating and distributing reports on their growing influence. They worked to make their posts seem more authentically American, and to create posts more likely to spread virally through the mechanisms of the social networks.

Facebook greeted the Mueller indictment as a kind of vindication. (As did President Donald Trump, who eagerly promoted some tweets by the company’s vice president of ad products that criticized media coverage of Russian ad spending on Facebook.) “We proactively disclosed the IRA activity to the Special Counsel, Congress, and the public, and have worked with them to give the public a fuller understanding of what occurred,” said Joel Kaplan, Facebook’s vice president of global policy, in a statement. “We’re grateful the US government is now taking this aggressive action against those who abused our service and exploited the openness of our democratic process.“

Kaplan added, “We know we have more to do to prevent against future attacks.” He added that the number of people working on security at Facebook would double this year, to 20,000. “We’re committed to staying ahead of this kind of deceptive and malevolent activity going forward,” he wrote.

In the meantime, the dark side of “developing the social infrastructure for community” is now all too visible. The tools that are so useful for organizing a parenting group are just as effective at coercing large groups of Americans into yelling at each other. Facebook dreams of serving one global community, when in fact it serves — and enables —countless agitated tribes. The more Facebook pushes us into groups, the more it risks encouraging the kind of polarization that Russia so eagerly exploited.