Mark Zuckerberg Reuters/Robert Galbraith

Facebook brand pages are suddenly getting a lot less traffic, and it's threatening the entire social media marketing industry.

Earlier this month, Facebook changed the algorithm that determines which "stories" show up on the News Feed.

The News Feed is that center column you see when you go to Facebook.com or open your Facebook app.

Lots of industry insiders noticed the change and said it would screw over a few companies that depend on Facebook.

These industry insiders were right, but they had the wrong victims in mind.

They also may have missed the true motive behind Facebook's change.

In a December 2 blog post, Facebook said it would start to put more articles from publishers in the News Feed, ranking them by how much your friends have been clicking on them.

Facebook said it would favor "high quality articles" over "the latest meme."

When the change went down, lots of people assumed it would be terrible news for publishers that write a lot about "memes" and publish other "viral" content – publishers like Buzzfeed and Upworthy. Then AllThingsD's Mike Isaac reported that the Facebook executive in charge of News Feed, Chris Cox, has a personal distaste for those two sites. It seemed like their doom was imminent.

But it turns out Buzzfeed and Upworthy aren't the companies getting immediately screwed over by the change.

The people already getting hosed are "social media marketers" – an industry of people who run Facebook pages for big brands.

A week or so after Facebook made its changes, one social media marketing agency, Ignite, analyzed 689 posts from 21 brand pages. Ignite found that in just one week, the number of people who saw posts from those brands declined by 44% on average, "with some pages seeing declines as high as 88%."

So what was Facebook's News Feed tweak actually about?

To answer that question, you have to remember exactly what Facebook brand pages are.

They are a free way for companies to use Facebook to talk to their customers and gin up more sales.

They are free ads.

Well, they are almost free. Even free marketing requires management. Someone has to come up with the creative. Someone has to make sure the creative is working – that the brand is getting lots of "likes." And that's why "social media marketing" agencies, like Ignite, exist.

So here's a hypothesis: Facebook's News Feed tweak wasn't only about getting "higher quality content" in the News feed. It was as much about reducing the reach of free ads on Facebook. Now, if a brand wants exposure on Facebook, it's going to have to buy it from Facebook.

There will be two pieces of fallout from the change:

Social media marketing firms are going to have to figure out new core competencies or they are going to go out of business.

Facebook's Wall Street analysts will soon begin to notice that the company figured out yet another way to bolster revenues, even as it stops stuffing more ads in the News Feed.

While they are screwed in the short term, don't worry too much about the people in the social media marketing industry.

They have been through a similar trial before.

Google once went through a period where it adjusted its search algorithms so that companies couldn't just game the system to get their pages to the top of search results pages.

Google's message to companies was: Quit paying "search engine optimizers" to get to the top of Google. If you want sales from our users, do a simple ROI calculation, and buy as many Google ads as you can afford; you won't regret it.

When that happened, tons of search engine optimization, or "SEO," experts who'd spent their days studying Google, suddenly became social media marketing, or "SMM," experts who spent their days studying Facebook.

Now, they're already on to Snapchat.