Mark Zuckerberg's admission he was among those caught up in the Cambridge Analytica scandal shows Facebook lost control of users' data, an Australian privacy expert says.
During a second, extended round of questions from US lawmakers overnight, the billionaire founder of the social media giant revealed his own personal information may have been shared with the controversial British political consultancy agency.
Anna Johnston, director of Sydney legal firm Salinger Privacy, says it is absurd the man running the company didn't organise his own privacy settings to protect his personal data.
"(It) suggests that a: users are not in control, and b: Facebook itself has lost control of the data," the former NSW deputy privacy commissioner told AAP on Thursday.
Facebook has blamed a company co-founded by Cambridge University researcher Aleksandr Kogan for improperly collecting data on up to 87 million people around the world, including 311,127 Australians.
Some or all of the information was then licensed to Cambridge Analytica, which is based in Britain.
The scandal has prompted privacy commissioners in Australia, the UK and Canada to open investigations into Facebook.
Mr Zuckerberg has insisted users have control of the information they post on Facebook.
He told the congressional inquiry giving people control of their information and how they want to set their privacy was "foundational to the whole service".
"The difference between surveillance and what we do... is extremely clear, which is that, on Facebook, you have control over your information," he said.
However, he acknowledged regulation of social media companies is "inevitable" to help protect users.
Some US lawmakers were particularly concerned about the way Facebook tracks users around the web with the use of like and share buttons, and a hidden script called Facebook Pixel.
Zuckerberg estimated more than 100 million web pages had at one stage contained a like button, and promised to provide more information on the matter at a later stage.
Many web pages are riddled with Facebook and Google trackers, according to a report released last year from German-owned web privacy company Ghostery.
Google's tools were present in three in every five pages loaded, while Facebook's trackers were in a quarter of all page loads.
"When Zuckerberg and chief operating officer Sheryl Sandberg keep talking about users being in control, sure, that true," Ms Johnston said.
"But that's not the only information that Facebook has about you.
"The rest of it you have no control over it and that's actually the part that is most likely to be damaging.
"The Cambridge Analytica scandal has simply exposed in a dramatic way what many people have been pointing out for many years - that this is Facebook's business model."