Mark Zuckerberg testified at a joint hearing of the Senate Judiciary and Commerce committees Tuesday, in the first of two congressional hearings for the Facebook founder and CEO.

The company is facing questions following reports that research firm Cambridge Analytica improperly gained access to the personal data of as many as 87 million Facebook users.

He's scheduled to testify before the House Energy and Commerce Committee Wednesday. His prepared remarks for that hearing were released Monday. Zuckerberg is likely to face tough questions on user privacy, foreign meddling on the site and abuse of social media tools.

Here is CNBC's coverage of Zuckerberg's testimony: