10 million people saw Russian ads on Facebook around the 2016 presidential election

Roughly 10 million Facebook users in the United States saw advertisements purchased by Russian-backed sources before and after the 2016 U.S. presidential election, the company revealed on Monday, as it admitted it faces a daunting task in balancing foreign election meddling with sincere public debate.

Forty-four percent of those ads, 3,000 in total — some of which sought to stoke racial, religious or other social tensions — had been viewed before Election Day, according to Facebook, while 56 percent of the ads were viewed after the race concluded. And about 25 percent of the ads hadn’t been seen by anyone at all.

The admission from Facebook comes hours after it shared copies of the suspect ads with congressional lawmakers, who are investigating Russian interference in the U.S. presidential contest. Those members of Congress — on both the House and Senate Intelligence Committees — also expect Facebook and its counterparts, Google and Twitter, to testify at public hearings in the coming weeks.

Ten million viewers is not a trivial number: It’s essentially equal to the population of the state of Michigan, though Facebook did not offer a geographical breakdown of the users who saw those ads.

In revealing its findings, though, Facebook explicitly acknowledged the difficulty it faces in moderating content on its platform.

The social giant allows for precise ad targeting based on demographics, like race and gender, which Facebook stresses is essential for connecting people with content that interests them. But it has long been the stuff of controversy, at times allowing malefactors to target users with terms like “Jew hater.”

To that end, Facebook also admitted Monday that its own system has been abused. And it stressed the difference between inflammatory political content and objectionable yet acceptable debate is sometimes hard for it to address

“Even when we have taken all steps to control abuse, there will be political and social content that will appear on our platform that people will find objectionable, and that we will find objectionable,” wrote Elliot Schrage, Facebook’s vice president of policy and communications.

“We permit these messages because we share the values of free speech — that when the right to speech is censored or restricted for any of us, it diminishes the rights to speech for all of us, and that when people have the right and opportunity to engage in free and full political expression, over time, they will move forward, not backwards, in promoting democracy and the rights of all,” he said.

Schrage said that Facebook requires “authenticity regardless of location,” explaining: “If Americans conducted a coordinated, inauthentic operation — as the Russian organization did in this case — we would take their ads down, too.”

“However, many of these ads did not violate our content policies,” he continued. “That means that for most of them, if they had been run by authentic individuals, anywhere, they could have remained on the platform.”

Amid the scrutiny, Facebook also announced on Monday a series of steps to improve its practices for vetting advertisements — including a pledge to hire 1,000 new workers to review submissions, and a commitment to apply its machine-learning acumen toward studying divisive political content.

This article originally appeared on Recode.net.