A former Facebook employee has told US lawmakers that the company's sites and apps "harm children, stoke division and weaken our democracy".
Frances Haugen, a 37-year-old former product manager turned whistleblower, heavily criticised the company at a hearing on Capitol Hill.
Facebook has faced growing scrutiny and increasing calls for its regulation.
Founder Mark Zuckerberg hit back, saying recent coverage painted a "false picture" of the company.
In a letter to staff, he said many of the claims "don't make any sense", pointing to their efforts in fighting harmful content, establishing transparency and creating "an industry-leading research program to under these important issues".
"We care deeply about issues like safety, well-being and mental health," he said in the letter, made public on his Facebook page. "It's difficult to see coverage that misrepresents our work and our motives."
Facebook is the world's most popular social media site. The company says it has 2.7 billion monthly active users. Hundreds of millions of people also use the company's other products, including WhatsApp and Instagram.
But it has been criticised for several things - from failing to protect users' privacy to not doing enough to halt the spread of disinformation.
Ms Haugen told CBS News on Sunday that she had shared a number of internal Facebook documents with the Wall Street Journal in recent weeks.
Using the documents, the WSJ reported that research carried out by Instagram showed the app could harm girls' mental health.
This was a theme Ms Haugen continued during her testimony on Tuesday. "The company's leadership knows how to make Facebook and Instagram safer, but won't make the necessary changes because they have put their astronomical profits before people," she said.
She criticised Mark Zuckerberg for having wide-ranging control, saying that there is "no one currently holding Mark accountable but himself."