Facebook groups are "dangerous" and use algorithms that "take people who have mainstream interests and push them to extreme interests", a whistleblower has told MPs.
Frances Haugen said the groups can become "echo chambers" reinforcing and radicalising people's opinions.
For example, users with left wing opinions can be pushed to the radical left, while those looking for healthy recipes can be pushed to anorexia content.
Ms Haugen, a former employee in the company's civic integrity unit, has been giving evidence to MPs considering the draft online safety bill.
Among her evidence, she also told the committee that Facebook sees safety as a "cost centre" and that it "unquestionably" makes online hate worse.
She previously appeared before the US Senate and her testimony in the UK comes as fresh stories appeared on Monday based on the thousands of documents she leaked.
Facebook boss Mr Zuckerberg has rejected her claims, saying her attacks on the company were "misrepresenting" the work it does and that it "cares deeply about issues like safety, well-being and mental health".