Facebook shares fell as much as 8 percent on Monday after reports that Cambridge Analytica mined the data of over 50 million users of the social network without their permission.
Cambridge Analytica worked on Facebook ads for President Donald Trump during the 2016 election. It was initially funded by conservative billionaire Robert Mercer and led by former Trump advisor Steve Bannon.
Whistleblower Christopher Wylie revealed the alleged role played by Cambridge Analytica in obtaining data from Facebook users to The Observer newspaper in the U.K. and The New York Times.
Both newspapers ran stories Sunday outlining how Cambridge academic Aleksander Kogan made an app called "thisisyourdigitallife" that prompted users to answer questions for a psychological profile. This data was then shared with Cambridge Analytica, allegedly without Facebook's consent. The app was blocked by the social network in 2015.
Facebook said that all the parties that had received the data said it had been destroyed, although the company said it had received reports that not all the data was actually deleted.
"We are moving aggressively to determine the accuracy of these claims. If true, this is another unacceptable violation of trust and the commitments they made," Paul Grewal, deputy general counsel at Facebook, said in a blog post Saturday.
Facebook said it has suspended Cambridge Analytica from its platform.
According to the social network, Kogan "lied" to Facebook by passing the data along to Strategic Communication Laboratories (SCL) and Cambridge Analytica — an SCL affiliate — without informing users.
Both The New York Times and The Observer called this a data "breach." Facebook said that it is not a breach.
"The claim that this is a data breach is completely false. Aleksandr Kogan requested and gained access to information from users who chose to sign up to his app, and everyone involved gave their consent. People knowingly provided their information, no systems were infiltrated, and no passwords or sensitive pieces of information were stolen or hacked," Grewal said.
Cambridge Analytica has denied violating Facebook's terms of service.
Senators Amy Klobuchar, D-Minn., and John Kennedy, R-La., called Monday for the Senate Judiciary Committee to bring Mark Zuckerberg and other tech CEOs before congress to testify following the report.
--CNBC's Matt Rosoff and Christina Farr contributed to this report.