Facebook may be the latest Silicon Valley company to be accused of political bias this election cycle. But it’s not the first. And it most certainly won’t be the last.

The US Senate Commerce Committee is asking Facebook for answers after a Gizmodo report alleged that the company’s news curators have been deliberately suppressing conservative news from surfacing in its Trending Topics.

In a letter sent to CEO Mark Zuckerberg yesterday, the committee asks for a thorough explanation of how Trending Topics work, what the guidelines are for determining which topics to include or remove, under what circumstances a curator might inject a story into Trending Topics, and more.

Tom Stocky, Facebook’s head of Trending Topics, has already responded to the original report, writing that reviewers are trained to “disregard junk or duplicate topics, hoaxes, or subjects with insufficient sources.” Still, questions abound about just what exactly constitutes “junk.”

It’s a sticky situation for Facebook, which is working to make a name for itself in news. And yet the social networking giant is not alone in struggling to walk a hair-thin line down the middle in what is already a historically polarized election season. Earlier this year, Twitter was accused of censoring the hashtag #WhichHillary, which Bernie Sanders supporters and other Hillary Clinton opponents were using to highlight inconsistencies in the Democratic frontrunner’s record. Twitter, too, denied those accusations.

The fact is, tech companies are no longer novelties in the world of news and politics. Far from it. Today, they are among the top sources of news for most Americans. In response, both Facebook and Twitter have started using human curators to monitor and sift through the oceans of information shared on their sites.

In doing so, these companies have started acting overtly like traditional publishers. But in effect they’ve always been in the publishing business (see also Google News). Though they may be more comfortable identifying as platforms, seemingly neutral utilities closer to e-mail or the telephone than The New York Times, editorial judgment is built into what the biggest tech companies do. Forget about Trending Topics: the act of enforcing—or choosing not to enforce—any kind of standards at all is an editorial decision. Algorithms themselves act as a reflection of their creators’ judgment in the search results they generate and the News Feed items they surface, automating the act of editorial decision-making. The Trending Topics controversy isn’t an anomaly; deciding what does and doesn’t get attention online is at the core of what these companies do. And as any editor knows, making these calls becomes especially fraught during a contentious election year.

Internal Biases

As tech companies become central to the dissemination of news, the biases of their leaders become subject to greater scrutiny, as they would with any publisher. And it’s a simple fact that San Francisco and Silicon Valley just tend to lean left. Apple CEO Tim Cook was on the front lines of the fight for marriage equality. More recently, PayPal pulled the plug on its plans to open a new operation in North Carolina after the state passed its controversial anti-transgender “bathroom bill.”

Tech companies are no longer novelties in the worlds of news and politics.

Zuckerberg himself, as well as LinkedIn founder Reid Hoffman, are key funders of the pro-immigration reform lobbying group FWD.us. Tesla founder Elon Musk is an outspoken opponent of the fossil fuel industry. Alphabet Chairman Eric Schmidt was actively involved in President Obama’s 2012 campaign and now invests in companies that work with the Clinton campaign.

The list goes on. As these tech leaders become more outspoken about the issues that matter to them, the pressure to balance their personal beliefs and the neutrality of their platforms mounts.

But Trump, whose disapproval rating is historically high, poses a particular problem for businesses. He’s so divisive that some employees and customers of these tech businesses are demanding Trump bans. This week, for instance, a petition signed by a group of Amazon shareholders launched online asking Amazon CEO Jeff Bezos to pull the plug on all of its Trump merchandise. In March a group of Facebook employees used an internal poll to ask Zuckerberg what responsibility the company has to prevent “President Trump in 2017.”

Media Moguls

On stage at one of its biggest events of the year, Zuckerberg not-so-subtly criticized the billionaire’s immigration plans. Just weeks later, Facebook made headlines when it announced it would still sponsor the Republican convention in July, along with Google and Microsoft, despite the fact that Trump is the presumptive nominee. And on Monday, Facebook investor and board member Peter Thiel appeared on Trump’s California delegate slate.

The problem here is not that so many powerful people in Silicon Valley are mixing business and politics or that the Silicon Valley companies they run are now becoming purveyors of news. History is full of influential news moguls with transparent political leanings: William Randolph Hearst was a Democratic member of the House of Representatives. Joseph Pulitzer was a Democratic Congressman. Rupert Murdoch is a well-known conservative.

The problem is that the people who use Facebook and Google, LinkedIn and Amazon, expect that these services are making decisions independent of human judgment—that the machines can rise above the differences that divide us. Silicon Valley makes products that can show new routes to avoid traffic, even in a familiar town. They construct algorithms that can recommend a new album you might like out of a sea of thousands. They build software that can give us personalized weight loss tips based on daily activity levels. These are seemingly objective undertakings, and Silicon Valley itself thrives on the impression that its software and hardware are inherently disinterested arbiters of information.

When that turns out not to be the case, people feel betrayed. If tech companies are now playing the role that traditional publishers have for centuries, then they need to begin having the same conversations about transparency and disclosure, ethics and fairness. With election season pressure rising, it looks like those conversations may happen sooner than later.