WASHINGTON — Facebook said Wednesday that the personal information of up to 87 million people, most of them Americans, may have been improperly shared with Cambridge Analytica, a political consulting firm connected to President Trump during the 2016 election.

The new figure, roughly equivalent to a quarter of the population of the United States, is substantially greater than the previous estimate of how many users’ information Cambridge Analytica harvested. The number had been put at more than 50 million users.

Facebook released the revised figure as part of an extended statement about changes it is making to how it handles personal data. The company said it would start telling users on April 9 about whether their information might have been shared with Cambridge Analytica.

“We wanted to put out the maximum number of people who could have been affected,’’ Mark Zuckerberg, Facebook’s chief executive, said on a call with reporters.

Cambridge Analytica responded to Facebook’s revised estimate with a statement in which the firm said it had licensed data for no more than 30 million users of the social network, that none of the data had been used in the firm’s work on the 2016 election, and that it believed at the time that the data had been legally obtained.

The sharing of user data has unleashed a torrent of criticism against Facebook, the world’s largest social network.

The company is under investigation by the Federal Trade Commission over whether it violated a 2011 agreement meant to protect users’ privacy. Investors have been fleeing the company as well, with its stock falling sharply in recent weeks.

Earlier Wednesday, lawmakers in Washington said that Zuckerberg would testify before the House Energy and Commerce Committee on April 11 about the company’s handling of sensitive user data.

Zuckerberg is also expected to appear before at least one Senate committee next week.

“This hearing will be an important opportunity to shed light on critical consumer data privacy issues,’’ said Representatives Greg Walden, Republican of Oregon, and Frank Pallone, Democrat of New Jersey, of the House committee. “We appreciate Mr. Zuckerberg’s willingness to testify before the committee.’’

After the revelations about Cambridge Analytica emerged, Facebook unveiled a new privacy center that streamlines the settings people use to choose what type of data Facebook collects on them and shares with advertisers. In announcing the center, Facebook said its goal was to introduce more transparency about the platform to users.

Among the changes announced Wednesday, Facebook said it would restrict the data to which third-party app developers would have access.

The company is also racing to change the service to comply with new data privacy rules that will take effect in the European Union next month and limit what type of user data companies can collect and how they can store it.

Zuckerberg, responding to a question about the new European rules, said during the call on Wednesday that the changes the company was making would apply worldwide, not just in Europe.

Facebook’s problems stretch back before the reports about Cambridge Analytica, to earlier investigations into how Russian actors infiltrated the platform by placing ads and posts to disrupt the 2016 election. At the time, Zuckerberg dismissed the idea of foreign interference on Facebook as a “crazy idea.’’

Since then, the company has been the focus of investigations by law enforcement and congressional committees that are delving into Russian interference in the election. Facebook now acknowledges that its platform was used by agents to influence voters.

In the past couple of weeks, the company has been on something of a public relations offensive before Zuckerberg’s expected appearance before Congress. Few executives draw the same interest as Zuckerberg, a 33-year-old billionaire who has connected 2 billion people to a social media platform he helped create as a college student.

His session with reporters Wednesday was a rarity for him: He typically appears in such settings only on quarterly financial calls. Sheryl Sandberg, Facebook’s chief operating officer, is set to be interviewed Friday by Fox News, “PBS NewsHour,’’ NBC’s “Today’’ show, and Bloomberg. Sandberg will be interviewed remotely from California.

Facebook said the efforts were meant to show how seriously it takes the intense criticism it has faced over its handling of user data. The company said it wants to explain specific steps it is taking to correct its site’s weaknesses.

In Washington, Facebook employees and public relations firms retained by the company have talked to regulators and congressional staff members about new privacy measures, including updates to privacy policies that are intended to make them easier to understand.

On Wednesday, Zuckerberg provided a preview of what he will tell Congress next week. He said Facebook was “an idealistic and optimistic company.’’

But he admitted that the company had committed serious errors by not ensuring that robust safeguards were in place for users.

“We didn’t take a broad enough view of what our responsibility is,’’ he told reporters. “That was a huge mistake.’’

Asked if he should still be leading the company, he said, “Yes. Life is about learning from the mistakes and learning what you need to do forward.’’

Terrell McSweeny, a Democratic member of the Federal Trade Commission, declined to comment on the planned updates and how they might affect the commission’s investigation into Facebook’s privacy practices.

But she said that Zuckerberg has a big task ahead of him in Washington.

“I think it is important for Zuckerberg to clearly explain how Facebook plans to earn back consumer trust,’’ McSweeny said. “Consumers need reassurance that their data are not being misused.’’