Yet the disclosure falls short of notifying more than 100 million other users who came across the pages’ ads and posts, and may have liked, shared or commented on them. And it provides little information to those users who did follow the pages.

By the end of the year, users will have access to a tool to check if they followed any of the pages, which were designed to look like they were run by Americans but were actually created by a single pro-Kremlin firm called the Internet Research Agency. Sen. Richard Blumenthal (D., Conn.) this month following a congressional hearing asked Facebook, Twitter Inc. and Alphabet Inc.’s Google to tell users who saw posts by the Internet Research Agency, and asked for a reply by Wednesday.

Facebook’s disclosures, however, will only reach a fraction of the more than 120 million users who saw the posts, the company said. A Facebook spokesman said it is too difficult to reach all affected users, in part because it can’t reliably identify who came across the content.

Facebook’s disclosure to users is “really the bare minimum,” said Elevation Partners co-founder Roger McNamee, an early Facebook investor and adviser who has become a critic of social media’s effect on democracy. Facebook’s “reluctance to both accept responsibility for what happened in 2016 and, more importantly, to take steps to prevent similar things happening in the future is just both horribly disappointing and really dangerous.”

Sen. Blumenthal requested Facebook reach all users who came across any content from the Internet Research Agency and tell them “exactly what content they saw so they can understand and evaluate what they may see in the future,” according to a letter he sent to the company.

Sen. Blumenthal on Wednesday called Facebook’s move “an encouraging step” and added, “I hope that Google and Twitter will follow Facebook’s lead.”

U.S. intelligence agencies, lawmakers and tech firms say Russian actors aimed to influence the presidential election last year by pushing false and misleading information and sowing division among American voters. One pillar of the Russians’ strategy, which U.S. intelligence agencies say was directed by the Russian government, was to create fake accounts on popular U.S. tech sites, including Facebook, Instagram, Twitter and Google’s YouTube. On Facebook and Instagram, the Russian accounts ran thousands of ads as a way to amass followers to their pages.

Google and Twitter declined to comment on whether they plan to tell users who came across such Russian content on their sites. Google told Congress Russia-linked accounts created 1,108 YouTube videos that were viewed about 309,000 times, while Twitter said 36,746 Russian bots tweeted about the election, which were viewed about 288 million times.

Facebook told Congress that between June 2015 and August 2017, roughly 126 million Americans came across at least one of the more than 80,000 posts from Internet Research Agency’s 120 Facebook pages.

Facebook said 11.4 million Americans saw nearly 3,400 Facebook ads purchased by the Internet Research Agency. And Facebook said the agency created another 170 pages on Instagram.

Facebook said it would enable users to soon check if they followed any of the Internet Research Agency pages by going to facebook.com/actionplan. Users would see a list of any Internet Research Agency pages they followed and the date they began following.

Mr. McNamee said Facebook should also reach out to affected users directly rather than require them to seek out the information on an obscure Facebook page.

“The polarization we have in the country today has made tens of millions of people resistant to facts and Facebook is in a unique position to break through that resistance,” Mr. McNamee said. “It’s really important that they reach out to every single person who was touched and say, ‘We—meaning Facebook—were manipulated’.”

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