Facebook Inc. CEO Mark Zuckerberg apologized and admitted the company has made “mistakes” in the handling of the Cambridge Analytica scandal that has sent its stock plummeting.

In a lengthy Facebook post, Zuckerberg addressed the controversy that a London-based data analytics firm collected the data from 50 million Facebook users without their permission. The firm has ties to the President Donald Trump’s campaign, according to the Los Angeles Times. 

“We have a responsibility to protect your data, and if we can’t then we don’t deserve to serve you,” he wrote. “The good news is that the most important actions to prevent this from happening again today we have already taken years ago. But we also made mistakes, there’s more to do, and we need to step up and do it.”

Zuckerberg laid out the company’s plans on how they will protect user data from third-party applications in the future.

“We will investigate all apps that had access to large amounts of information before we changed our platform to dramatically reduce data access in 2014, and we will conduct a full audit of any app with suspicious activity,” he said. “If we find developers that misused personally identifiable information, we will ban them and tell everyone affected by those apps.”

He also plans to be more transparent with members of the site about which applications are using user data by moving tools to the Facebook’s News Feed.

Investors in the social media giant haven’t reacted well to the news resulting in Facebook losing $50 billion in valuation, according to Quartz.

U.S. lawmakers want Zuckerberg to testify before Congress about the controversy and how  Cambridge Analytica was able to use the data unlawfully.

He told CNN he would be “happy” to testify on Capitol Hill.