Facing a growing backlash over the role that fake news and hate speech may have played in the recent U.S. presidential election, three of Silicon Valley’s largest tech companies have announced plans to try to address the problem.

Google and Facebook have said they will update their advertising policies to target misleading ads that direct people to fake news sites. Meanwhile, USA Today reported that Twitter has closed the accounts of several notable alt-right personalities.

The news comes as the nation remains in shock over Donald Trump’s upset victory last week.

Over the weekend, Facebook CEO Mark Zuckerberg insisted that the amount of fake news on the platform remains tiny and could hardly have swayed the outcome. Still, the Trump campaign is now bragging about how Facebook’s platform helped them win the election.

Google was drawn into the fray this week when an obviously fake news site that incorrectly claimed Trump had won the popular vote shot to the top of search results.

All these companies face a massive task, and it’s not clear how much they can really do to stem fake news, hate speech, and bullying. At the same time, their brands and their appeal to advertisers, not to mention users, could be at risk if the problem is ignored.