WASHINGTON — The CEOs of Facebook, Twitter and Google will tell lawmakers on Thursday they have taken significant steps to curb the spread of disinformation, hate speech and extremist content on their platforms, according to written testimony posted online Wednesday ahead of a congressional hearing.

Facebook CEO Mark Zuckerberg, in his testimony, wrote that he supports reforming a key internet liability law to require companies to show they can identify unlawful content and remove it.

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Advocates of stronger federal antitrust action against the companies hope to see lawmakers pledge to resolve the underlying financial incentives that drive dangerous content.

“Policy-makers themselves [need to] recognize and acknowledge that the toxic business model and monopoly power of the tech platforms are a result of their failure,” said Sarah Miller, executive director of The American Economic Liberties Project, and co-chair of Freedom From Facebook and Google.

“At the end of the day, it is up to policy-makers to solve these problems, rather than deflect responsibility back to CEOs, who time and time again won’t and can’t build platforms that are safe for democracy and healthy for society,” Ms. Miller said.