If your data was harvested, you will see a message that reads: "A friend of yours did log in". Among the numerous items on the agenda were data ownership and access and privacy rights.
In response, he initially said no, but reiterated Facebook's new investigation into third-party apps on the platform was seeking to discover if there had been any other cases of misuse.
But Zuckerberg continually dodged the question about advertisers by talking about what users are sharing.
His questioner, Representative Ben Lujan, a New Mexico Democrat, said the practice creates "shadow profiles". Facebook's main product hasn't changed that much in recent years, so perhaps, like Zuckerberg, they're reminiscing about a time when the company was run out of a Harvard dorm room and the key feature was the "poke".
On Tuesday, Zuckerberg took questions from 44 senators seeking an explanation for how Facebook failed to prevent a data-mining company from gathering personal information on 87 million users and whether the company does enough to protect users' data. Blackburn asked Zuckerberg as he testified before the House Energy and Commerce Committee.
"That's why, every single time that you share something on Facebook or one of our services, right there is a control in line, where you control who you want to share with, because I don't just think that this is about a terms of service". The nice thing about this is that it also provides Facebook with a great incentive to make its privacy settings easy to use.
The first question lobbed at Zuckerberg on Wednesday was about whether Facebook is a media company. "What we know now is that Cambridge Analytica improperly accessed information by buying it". Why wasn't explaining what Facebook does with users' data a higher priority for you as a co-founder and now as CEO?
The social media company, he said, is not aware of any specific groups like that, that have engaged in the white supremacist rally in Charlottesville previous year. But that doesn't mean it doesn't profit from it.
Despite the probing yesterday and today, Mr Zuckerberg has not buckled under pressure and the share price of Facebook rose 4.5 percent after yesterday's hearing, although shares were slightly down today.
It added $21.3 billion to Facebook's market value and $3 billion to Zuckerberg's personal net worth, Fortune reported. And he did finally say, yes, we do track users when they log off, both for security purposes and for advertising purposes.