NEW YORK — Breaking five days of silence, Facebook CEO Mark Zuckerberg admitted mistakes and outlined steps to protect user data in light of a privacy scandal involving a Trump-connected data-mining firm.

Zuckerberg said Wednesday that Facebook has a “responsibility” to protect its users’ data and if it fails, “we don’t deserve to serve you.”

But Zuckerberg stopped short of apologizing.

And he wrote “what happened” instead of “what we did,” leaving Facebook one step removed from responsibility.

Richard Levick, chairman of the crisis-management firm Levick, gave Zuckerberg’s response a “B-” grade, in part because of how late it came.

Zuckerberg and Facebook’s No. 2 executive, Sheryl Sandberg, had been quiet since news broke Friday that Cambridge Analytica may have used data improperly obtained from roughly 50 million Facebook users to try to sway elections. Cambridge’s clients included Donald Trump’s campaign.

Facebook shares have dropped some 8 percent, lopping about $46 billion off the company’s market value, since the revelations were first published.

Even before the scandal broke, Facebook has already taken the most important steps to prevent a recurrence, Zuckerberg said. For example, in 2014, it reduced access outside apps had to user data. However, some of the measures didn’t take effect until a year later, allowing Cambridge to access the data in the intervening months.

Zuckerberg acknowledged that there is more to do.

In a Facebook post on Wednesday, Zuckerberg said it will ban developers who don’t agree to an audit. An app’s developer will no longer have access to data from people who haven’t used that app in three months. Data will also be generally limited to user names, profile photos and email, unless the developer signs a contract with Facebook and gets user approval.

In a separate post, Facebook said it will inform people whose data was misused by apps. Facebook first learned of this breach of privacy more than two years ago.

The company said it was “building a way” for people to know if their data was accessed by “This Is Your Digital Life,” the psychological-profiling quiz app that researcher Aleksandr Kogan created and paid about 270,000 people to take part in. Cambridge Analytica later obtained information from the app for about 50 million Facebook users, as the app also vacuumed up data on people’s friends — including those who never downloaded the app or gave explicit consent.

Cambridge has shifted the blame to Kogan, which the firm described as a contractor. Kogan described himself as a scapegoat.