Facebook chief Mark Zuckerberg has admitted the social network "made mistakes" over the Cambridge Analytica data scandal.
In his first comments since the company disclosed the misuse of personal data, Zuckerberg said Facebook would work to "step up" and address its failings.
He pledged to introduce changes that would make it more difficult for third-party applications to harvest Facebook user data.
Zuckerberg said a "breach of trust" had occurred between Cambridge Analytica and Facebook.
Over the weekend it was revealed that the data analytics firm had gained improper access to data on 50 million Facebook users in 2014 to profile and target them with political ads.
Cambridge Analytica had collected the data through an app that asked users to undertake a personality test for research purposes.
Around 270,000 people agreed to take the test, but Facebook's terms of service and API at the time allowed the firm to also collect the data of the participants' friends - or the information of more than 50 million people.
Zuckerberg today admitted there had also been a breach of trust between Facebook and its users.
"I started Facebook, and at the end of the day I'm responsible for what happens on our platform," he said in a Facebook post.
"We have a responsibility to protect your data, and if we can't then we don't deserve to serve you. I've been working to understand exactly what happened and how to make sure this doesn't happen again."
He said the "most important" actions to prevent a similar reccurrence had been taken in 2014, when Facebook took measures to "drastically reduce" data access by third parties.
The company stopped allowing apps to access the data of a person's friends without their approval, closing the loophole that Cambridge Analytica had exploited.
Zuckerberg today pledged to investigate all apps that had access to large amounts of information before the 2014 changes.
Facebook will also conduct a full audit of any app with suspicious activity, and ban developers that don't agree to a thorough audit and that have been found to have misused personally identifiable information, he said.
The company will further restrict developers' access to data to prevent similar kinds of abuse, Zuckerberg said, and will require developers to obtain approval from a user to access their posts or other private data.
"While this specific issue involving Cambridge Analytica should no longer happen with new apps today, that doesn't change what happened in the past. We will learn from this experience to secure our platform further and make our community safer for everyone going forward," Zuckerberg said.
"I want to thank all of you who continue to believe in our mission and work to build this community together. I know it takes longer to fix all these issues than we'd like, but I promise you we'll work through this and build a better service over the long term."