“Looking ahead, we will continue to invest heavily in security and privacy because we have a responsibility to keep people safe,” Mr. Zuckerberg said on the call.

Other factors have also hurt Facebook’s number of users, Mr. Zuckerberg said, including tough European rules that went into effect in May to protect people’s online data. The legislation, known as the General Data Protection Regulation, cost Facebook about one million users in Europe, he said.

The shift in Facebook’s business fortunes follows a series of crises that began in late 2016 with the revelations that it had become a prime distributor of misinformation. That has since been exacerbated by questions over the company’s role in securing private user data, its effect on the democratic process and its commitment to stemming disinformation on the site.

Mr. Zuckerberg has had to appear in front of lawmakers, has apologized profusely and has tied himself into knots explaining what he will and will not allow to appear on the social network.

After these events, several senior leaders have departed Facebook, including a board director, its chief information security officer, and its vice president of communications, marketing and public policy. This week, Colin Stretch, who led Facebook’s investigation into Russian election interference and who testified before Congress last year on Facebook’s behalf, said he would leave the company by the end of the year.

Facebook is now gearing up to face one of its biggest tests to date: ensuring that no one meddles in the 2018 midterm elections through the social network.