“There may be heads on pikes by the end of this,” one agency executive said.

For more than five hours on Monday, while Facebook and Instagram were dark, David Herrmann fretted about ads.

Mr. Herrmann, a freelance media buyer, said that everyone he worked with relied heavily on the platforms, which soak up the bulk of the $80 million to $100 million in ad spending he manages each year.

One company that advertises exclusively on Facebook watched its revenue plunge 70 percent during the outage from the same period a week earlier, Mr. Herrmann said. Sales slipped 30 percent at another company, which spends $40,000 a day on ads.

“I was more or less checking Facebook consistently throughout the day, hoping it would come back,” he said. “But without clear direction from Facebook, we just had to wait.”
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The outage was an unpleasant reminder to many advertisers of Facebook’s powerful influence on their ability to do business.

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Ads fuel Facebook, with more than 10 million advertisers contributing more than 98 percent of its revenue. In the three months ending in June 30, it pulled in an average of $78 million in ad sales every six hours, much of it from small companies, organizations and individuals.
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But a deluge of criticism has caused many of Facebook’s customers to sour on the company. Frances Haugen, a former project manager for Facebook turned whistle-blower, testified before senators on Tuesday that the company was aware of the harms caused by its services, such as Instagram’s negative effects on teenage girls. Facebook has also faced advertiser outcry over its handling of hate speech, misinformation, privacy and more.

Graham Mudd, Facebook’s vice president for ads and business product marketing, wrote on Twitter on Monday that the outage affected Facebook’s ad platform and apologized “for the disruption this creates for our customers.” Facebook said later in the day that “advertisers were not and will not be billed for ads during the outage.”

Media buyers noted that Facebook went dark at the beginning of the most important period for many advertisers, as they begin holiday campaigns during a season that is expected to be complicated this year by supply chain struggles and pandemic restrictions.

“There may be heads on pikes by the end of this,” Cory Dobbin, the founder of the Aaron Advertising digital agency, wrote on Twitter.

Many businesses rely exclusively on Facebook to reach customers, Mr. Dobbin, who manages roughly $50,000 a day in advertising spending, said in an interview. The majority of his clients’ spending goes to Facebook, with the rest to Google, Snap and other platforms.

“The name of the game for many advertisers, if it wasn’t already, is diversification,” he said. “This is a perfect example of why you can’t rely on a single channel to bring in all of your revenue.”

He continued: “It’s just far too risky to rely on Facebook to be there for your business long term.”

Mr. Dobbin said he would be surprised if Facebook refunded advertisers.

“This is how Facebook works,” he said. “Always has been, likely always will be.”

The outage added to growing concerns about Facebook. Omnicom Media Group, which manages more than $30 billion in global marketing spending a year, sent an update to clients on Monday noting that Facebook had been in touch to defend itself after Ms. Haugen appeared on the CBS program “60 Minutes” on Sunday.

Omnicom had sent a message to clients addressing a series of articles from The Wall Street Journal about Facebook. The ad company pointed to a report that Facebook misled its oversight board by claiming that its XCheck program, which exempted at least 5.8 million high-profile users from normal enforcement rules, was used only sparingly and that data from the system could not be tracked.

“These discrepancies raise a more significant question about the level of accuracy and sincerity of Facebook’s responses to inquiries — both internal and external — on these types of matters,” Omnicom wrote in the note, which was obtained by The New York Times. “The problem with what has come to light in this report is that there is essentially a parallel justice system within Facebook that operates with no public accountability or transparency.”

Omnicom, which along with clients created a lobbying group last year to push tech companies to take more responsibility on their platforms, also used its note to pressure Facebook to improve content moderation abroad and strengthen its management of misinformation.

“Taken as a whole, when it comes to user controls, Facebook is in many respects still years behind competitors,” Omnicom wrote, though it added that it “will continue to partner closely with Facebook” and “will keep our clients informed.”

Mr. Herrmann, the media buyer, said advertisers would continue to be shackled to Facebook because of its enormous size and reach.

“It can and does still have massive implications across the media buying space, so it’s not going anywhere,” he said. “TikTok is coming up quickly, but nobody at scale does it as well as Facebook.”