WASHINGTON, April 4 (Xinhua) -- Facebook revealed Wednesday that outsiders took advantage of search tools on its platform, making it possible for them to collect personal information on most of its 2 billion users worldwide without the users' explicit permission.

Most of Facebook's 2 billion users may have had their personal data skimmed from the site by "malicious actors," the company said in a blog post by Chief Technology Officer Mike Schroepfer.

The revelation came amid rising acknowledgement by Facebook about its struggles to control data it gathers on users. The company also said Wednesday that data firm Cambridge Analytica, which was hired by U.S. President Donald Trump's campaign, had improperly gathered detailed Facebook information on 87 million people, up from a previous news media estimate of more than 50 million.

Facebook Chief Executive Mark Zuckerberg will testify before the U.S. House Energy and Commerce Committee on April 11, the panel announced Wednesday.

"This hearing will be an important opportunity to shed light on critical consumer data privacy issues and help all Americans better understand what happens to their personal information online," the panel's Republican chairman Greg Walden and top Democrat Frank Pallone said in a statement.

Zuckerberg said that he is willing to testify before Congress. The tech giant also hired forensic auditors to investigate whether Cambridge Analytica still had the data, according to reports.