Facebook's being accused of knowingly harming children's mental health - including directing young users to anorexia content.
Employee-turned-whistleblower Frances Haugen has told a US Senate committee that its platforms - including Instagram - also deliberately stoke political division.
She says the company's not making changes, as it's putting its "astronomical profits before people".
Facebook denies the allegations, and says it's "not true" it encourages bad content.
At the start of the hearing, Ms Haugen told US Congress: "I'm here today because I believe Facebook's products harm children, stoke division and weaken our democracy."
She added: "Left alone, Facebook will continue to make choices that go against the common good. Our common good.
"When we realised big tobacco was hiding the harms, that caused the government to take action. When we figured out cars were safer with seatbelts, the government took action.
"And when our government learned that opioids were taking lives, the government took action," she said, adding: "I implore you to do the same here today."
In particular she warned that there's nobody at the company holding CEO Mark Zuckerberg accountable, other than himself.
"Mark holds a very unique role in the tech industry in that he holds over 55% of all the voting shares for Facebook.
"There are no similarly powerful companies that are as unilaterally controlled," she said.
