Facebook apparently has a liberal bias. And as a monopoly that counts nearly three in five American adults as users, this is a potentially enormous problem for all of us.

The controversy began on Monday, when Gizmodo published an eye-opening investigation charging that Facebook "routinely suppressed conservative news." Essentially, the "trending" news section that pops up in the right-hand side of your Facebook feed isn't a coldly mathematical automaton, after all. Facebook does have a batch of actual computer algorithms for finding and compiling news stories that are trending among Facebook users. But once compiled, those stories go through a team of human "curators" before they go up on Facebook's news crawl. These curators have the authority to both turn down stories collected by the algorithms and "inject" other stories the algorithms don't catch. They then write a headline and a summary for the aggregated story and up the link goes on Facebook's trending section.

It's hard to understate what a powerful perch this trending section is. Facebook has 1.65 billion active monthly users. Many American publishing giants get more than half of their traffic from Facebook. If Facebook "curators" put their thumbs on the scale, it has enormous effects, for publishers and readers alike.

And indeed, anonymous former curators said that the trending section was curated with a liberal bias. "I'd come on shift and I'd discover that CPAC or Mitt Romney or Glenn Beck or popular conservative topics wouldn't be trending because either the curator didn't recognize the news topic or it was like they had a bias against Ted Cruz." One of them even compiled a list of the stuff that got turned down:

Among the deep-sixed or suppressed topics on the list: former IRS official Lois Lerner, who was accused by Republicans of inappropriately scrutinizing conservative groups; Wisconsin Gov. Scott Walker; popular conservative news aggregator the Drudge Report; Chris Kyle, the former Navy SEAL who was murdered in 2013; and former Fox News contributor Steven Crowder. [Gizmodo]

Facebook basically denies everything. We "have found no evidence that the anonymous allegations are true," said Tom Stocky, the Facebook executive in charge of trending topics. "Facebook does not allow or advise our reviewers to systematically discriminate against sources of any ideological origin and we've designed our tools to make that technically not feasible."

Now, even if the charges are true, Facebook's behavior isn't too far removed from the editorial practices of a newspaper or media site: Journalists regularly make decisions about what is worthwhile news, what sources can be trusted, and how to package it, all based on their own ultimately subjective judgment. You don't have to posit conscientious conspiracies or dictates from on high to explain how this can go wrong.

Still, this is not a good look for Facebook. We're not talking about a media outlet that's upfront about its ideological priorities. We're talking about a massive social media platform that has pitched its trending news aggregation as neutral activity derived from the collective wisdom of its massive base of users.

News organizations have to work hard to police themselves — and the main reason is competition. If you, the reader, don't like the way one media outfit curates the news for you, you can just go with another one. And hopefully the process of millions upon millions of readers making that same decision again and again, all the time, will impose a kind of crowd-wisdom discipline on the industry.

But Facebook isn't just another media outlet with a host of competitors. Facebook is a thing unto itself. It began as a social media site to share messages and pictures and videos and weird odds and ends, then evolved into an ad hoc way for people to get their daily news intake, and now seems to rapidly be transforming itself into a one-stop shop for everything its users do on the internet. Mark Zuckerberg himself has said he wants Facebook to be "the primary news experience people have."

Facebook's closest competitor in this space is arguably Twitter, but there's really no other player with any serious clout doing exactly what Facebook is doing. Perhaps Facebook did a slipshod job building a news curation team because they were under no pressure to do it better or more fairly.

It's worrying to imagine Facebook operating in a manner similar to natural monopolies like highway or utility construction. The first player into the market becomes the single dominating force in setting the nature and options of the market. Sixty-three percent of Facebook's 167 million American users said they used Facebook in 2015 to get news. And 49 percent of its users age 18 to 34 say it's "the most or an important way they get news."

Ultimately, there's no way to avoid unconscious bias and subjective values in the job of picking and writing news stories. As a society, we're okay with that if an outlet states its ideological leanings up front. And we're okay with it claiming neutrality if it has competitors to keep it honest; lots of outlets with lots of individual newsrooms, all jostling and hustling for readers.

But what happens if Facebook eats the internet? And then they're the only ones doing the curating? It should be a scary thought — and not just for conservatives. Liberals and leftists have long combated monopoly power, because of its capacity to exploit workers, to lock competitors out of the economy, and to warp the functioning of democracy.

This turned out to be one of those instances where the cultural predilections of the people with the centralized power lined up with some lefty preferences and values. But if left-wing politics are about fighting for the little guy, that obviously isn't always going to happen. Indeed, the unique and immensely powerful role in news curation that Facebook is carving out for itself should worry liberals, too.