Snap Inc. issued a warning on Thursday about a long-feared internet advertising meltdown that could occur in the normally busy fourth quarter.

Long feared because, ever since Apple Inc. announced upcoming privacy changes to the iPhone, many companies with internet advertising businesses have warned about the potential impact on their results.

In April, Apple updated its mobile operating system to give users the option of not sharing or tracking their data, making it more difficult for software developers to track users across apps and advertisers to target their ads. According to Snap’s comments on Thursday, the results aren’t pretty. The young social-media company, best known for its Snapchat app, attributed its $3 million third-quarter revenue shortfall to changes made by Apple to iOS. Furthermore, Snap executives expect fourth-quarter revenue to grow at a rate of only 19% to 20%, compared to 57 percent growth in the third quarter.

Snap shares fell more than 20% in after-hours trading, dragging down some of the biggest names in technology with them — Facebook Inc., Alphabet Inc., Twitter Inc., and Pinterest Inc. all saw their shares fall following the Snap news. For more than a year, Facebook, for example, has been warning about the effects of Apple’s changes, most recently in September, but investors have yet to see much change to Facebook’s very profitable business model, even amid all the other controversies at the social-media giant.

Facebook even launched a full-fledged PR assault on Apple last December, with full-page ads in three national newspapers declaring that Apple’s then-upcoming changes would harm small businesses’ ability to offer targeted advertising to consumers, as well as app developers’ ability to offer free content.

However, Snap co-founder and CEO Evan Spiegel, who has previously avoided specific attacks on Apple regarding the change, stated on Thursday that the privacy changes have proven to be more problematic than expected, mentioning that Apple has even changed the tools that advertisers have to measure the results of their ads.

“I think we really underestimated the tooling changes,” Spiegel said. “Advertisers have essentially used a set of really sophisticated tools to measure and optimize their campaigns for a long time now, so that allows them to test out a bunch of different creative and see what’s performing.” According to him, Apple’s changes “essentially rendered those tools blind.”

Nonetheless, Spiegel restrained his criticism of Apple, saying that the privacy changes were “important to the long-term health of the ecosystem” and something “we fully support.”

Aside from Apple’s massive changes, global supply chain and staffing issues are influencing the number of advertisements that businesses are beginning to run during the crucial holiday season.

“[Advertisers] don’t necessarily want to accelerate the field of products that they will have difficulty getting into the hands of customers, and that is somewhat broad,” Snap Chief Business Officer Jeremi Gorman told analysts on Thursday’s post-earnings conference call.

However, in response to an analyst question, Snap executives stated that the Apple changes were having the greatest impact and would affect the broader advertising ecosystem, as some companies have been warning about but have yet to quantify.

“So what you’re seeing when we go into Q4 is a full-quarter impact of those issues, on Q4, and you know the reason that we’re mentioning, you know, iOS 15, is that that’s going to continue to disrupt the advertising ecosystem,” said Snap’s chief financial officer, Derek Anderson.

Next week, Facebook, Alphabet’s parent company, and Twitter will all report earnings, and investors will be able to gauge how widespread those issues are based on their results, guidance, and comments on company conference calls. If any of those companies’ fourth-quarter outlooks are as bleak as Snap’s, investors can expect a massive downturn in internet ad companies. Many investors were attempting to get ahead of potential bad news on Thursday, which could have an impact on the reported mega-merger talks between PayPal Holdings Inc. and Pinterest.

If Snap’s warning is applicable to its competitors, the next few weeks could spell “ad-mageddon” for internet stocks and social-media companies.