A growing number of online publishers including giants like BuzzFeed have come to rely on Facebook for a significant part of their traffic—in some cases as much as 60% of it. Mostly, it’s a win-win relationship with Facebook FB providing reach and a share of advertising revenue in exchange for a supply of engaging material. But every now and then, we get a glimpse behind the curtain at just how much power that relationship gives Facebook, and the consequences if it changes its mind.

We got another one of those on Monday, courtesy of a report from news site Digiday about some new numbers from SimpleReach and SimilarWeb, online analytics companies. According to their latest surveys, the top 30 publishers in terms of visibility on Facebook have seen their traffic plummet by more than 30% since the beginning of the year.

While some publishers saw an increase in traffic from Facebook—including Vice Media and Refinery29—the majority saw it fall. And to compound the problem, the more reliant a media outlet was on the social network, the more they saw their traffic from Facebook drop this year. The Huffington Post saw a decline of more than 60%, and BuzzFeed saw its Facebook referrals fall by more than 40%.

Why did this happen? To put it simply, no one knows—perhaps not even Facebook. Something changed, but it’s unclear what. As I’ve tried to describe a number of times, the reality of the media’s relationship with the giant social network is that it holds all the cards. It controls access to its 1.5 billion or so users, and it does this by tweaking the algorithm that determines which updates appear in a particular user’s news feed.

It’s like Facebook not only controls the trucks and satellites and newsstands that supply your newspaper, but also the layout and selection of all the stories that appear inside it.

 

This creates a number of potential issues, including the fact that a surprisingly large proportion of Facebook users are apparently unaware that the social network filters what they see and when they see it. Facebook also routinely censors news for a number of reasons, which is (or should be) problematic for media companies who are entering into a close relationship with the company. And yet, it has such a massive amount of reach that publishers of all kinds have to use it, whether they want to or not.

As the survey results from SimpleReach and SimilarWeb show, however, turning your distribution strategy over to Facebook brings with it a number of significant risks, and one of the most obvious is that the social network will change the rules of the game without telling you. In other words, things that used to work for media companies might suddenly stop working, whether it’s “curiosity gap” style headlines or auto-play videos or photos of cats.

Facebook, meanwhile, suggested in a statement to Digiday that part of the problem is so many media companies and publishers are posting so much of their output to the network that everyone’s reach effectively declines:

“As the number of posts to Facebook has increased substantially over the past few months, there has been a corresponding increase in the amount of potential posts to show any one person, which impacts reach. In this newly competitive landscape, we’re seeing results vary by publisher: some are experiencing continued growth in referral traffic while others have seen declines.”

Whatever the cause, publishers have seen a version of this particular movie before: Several major newspapers, including the Washington Post and The Guardian, created social-reading apps and got millions of followers in 2012. Then Facebook changed the rules, and their news essentially disappeared from view, along with most of those new readers.

This helps explain why some are so cautious about Instant Articles, which is Facebook’s offer to publishers of a fast-loading mobile version of their pages, provided that content is hosted completely on Facebook (Time Inc., which owns Fortune, is a partner in that project.) That and a new feature/app called Notify look to some like a way of locking media providers and their stories and videos into the Facebook platform for good. But who benefits the most from this relationship?

The other nagging fear for media companies is that Facebook is essentially engaging in a large-scale bait and switch, by encouraging them to host all of their output on its platform, but then gradually turning off the traffic tap so that their reach declines. At that point, the social network can recommend a number of ways to boost the reach again—including by paying for promoted posts and other forms of advertising. Facebook would no doubt protest that it is doing nothing of the kind, but the fear remains.

You can follow Mathew Ingram on Twitter at @mathewi, and read all of his posts here or via his RSS feed. And please subscribe to Data Sheet, Fortune’s daily newsletter on the business of technology.