Facebook Inc received several tongue-lashings during U.S. congressional hearings this week, but the world's largest social network also got an assignment: Figure out how to notify tens of millions of Americans who might have been fed Russian propaganda.
US lawmakers and some tech analysts are pressing the company to identify users who were served about 80,000 posts on Facebook, 120,000 on its Instagram picture-sharing app, and 3,000 ads that the company has traced to alleged Russian operatives, and to inform them.
The posts from Russia were designed to divide Americans, particularly around the 2016 U.S. elections, according to Facebook, U.S. intelligence agencies and lawmakers. The Russian government has denied it tried to meddle in the elections.
"When you discover a deceptive foreign government presentation on your platform, my presumption, from what you've said today - you'll stop it and take it down," Democratic Senator Jack Reed told Facebook General Counsel Colin Stretch in the Senate Intelligence Committee hearing on Wednesday.
"Do you feel an obligation, in turn, to notify those people who have accessed that? And can you do that? And shouldn't you do that?" Reed asked.
Stretch responded that he was not sure Facebook could identify the people because its estimates have relied on modelling, rather than actual counts, but he did not rule it out.
"The technical challenges associated with that undertaking are substantial," Stretch said.
Critics of Facebook on social media and in media interviews have expressed skepticism, noting that the company closely tracks user activity such as likes and clicks for advertising purposes.
Facebook declined to comment on Thursday.
As many as 126 million people could have been served the posts on Facebook and 20 million on Instagram, according to company estimates.
Many of them will not believe they were manipulated unless Facebook tells them, said Tristan Harris of Time Well Spent, an organisation critical of advertising-based social media.
"Facebook is a living, breathing crime scene, and they're the only ones with access to what happened," Harris, an ex-Google employee, said in an interview on Thursday.
The 2.1 billion people with active Facebook accounts often get notifications from the service, on everything from birthdays and upcoming events to friend requests and natural disasters.
Shortly before 6 p.m. EDT on Thursday, more than 83,000 people had signed a Change.org online petition asking Facebook to tell users about the Russian posts.
Lawyers for Twitter Inc and Alphabet Inc's Google also said their companies would consider notifying customers.
The intelligence committee's vice chairman, Senator Mark Warner, drew an analogy to another industry.
"If you were in a medical facility, and you got exposed to a disease, the medical facility would have to tell the folks who were exposed," Warner said.
U.S. law includes a concept known as "post-sale duty to warn," which may require notifying previous buyers if a manufacturer discovers a problem with a product.
That legal duty likely does not apply to Facebook, said Christopher Robinette, a law professor at Widener University in Pennsylvania. He said courts would likely rule that social media posts are not a product but a service, which is exempt from the duty. Courts also do not want to interfere in free speech, he said.
Robinette added, though, that he thought notifications to users would be a good idea. "This strikes me as a fairly significant problem," he said.