The hearing is still rolling on, and Zuckerberg has barely broken a sweat after several hours of grilling.
After a nervy start, he almost seems to be enjoying himself.
To recap, he apologised to US senators for allowing several other firms to misuse his platform's personal data for political ends. He said it was his personal mistake.
On the subject of fake news, a contrite Zuckerberg said "one of my greatest regrets in running the company" was its slowness at uncovering and acting against disinformation campaigns by Russian trolls during the US election.
He said Facebook will investigate "tens of thousands" of apps to discover if any other companies have accessed data in a similar way to Cambridge Analytica.
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