Facebook (NASDAQ:FB) and Alphabet's (NASDAQ:GOOG) (NASDAQ:GOOGL) Google have experienced an uptick in usage as people turn to social media for information, entertainment, and solace amid the COVID-19 outbreak. 

But that isn't going to stop either company from seeing a slowdown in advertising sales. With travel coming to a halt, stores across the globe shuttered to foot traffic, and layoffs mounting, advertisers are reining in their ad spending.

How bad the loss will be depends on the length of the pandemic. But if China is any indication, it's going to be many billions of dollars.

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Ad spending across the globe is getting slashed

Prior to COVID-19, eMarketer had expected ad spending in China to come in at $121.13 billion, representing growth of 10.2%. It has now tempered that to 8.4% growth, with total ad spending in China projected to reach $113.67 billion. That marks the slowest growth rate in China since the firm started tracking ad spending back in 2011.

Print advertising is expected to take the biggest hit, but digital ad spending isn't immune. According to eMarketer, it previously expected 15.2% growth in digital ad spending totaling $86.3 billion. It's now projecting growth of 13%.

Keep in mind this forecast was made before the number of COVID-19 cases in the U.S. rose dramatically. That estimate could be at risk if the outbreak languishes in the U.S. and elsewhere and results in the cancellation of major sporting events. The NBA and NHL are among the professional sports leagues to end or cancel their seasons. Meanwhile, the Summer Olympic Games in Tokyo have been postponed, which eMarketer warned would result in a "meaningful" reduction in ad spending around the globe. That means billions of dollars in ads could be lost to the likes of Facebook, Google, and others.

It's already impacting microblogging platform operator Twitter (NYSE:TWTR), which this week pulled its guidance for the first quarter due to the coronavirus outbreak. Twitter cited a slowdown in advertising sales over the past few weeks. 

Uncertainty abounds 

Facebook's Chief Operating Officer Sheryl Sandberg highlighted the uncertainty in a recent interview with Bloomberg. The social media giant can continue to pay employees and maintain operations, she said, but the hit to advertising sales is a big unknown

This is not going to be business as usual, and the marketing industry is certainly going to see a real impact. I don't think anyone knows how big. So we're going to watch and look. 

While digital ads are more immune to a slowdown given the increase in online shopping and internet usage in the wake of COVID-19, they're also some of the easiest to adjust. With the economy in a tailspin, advertisers are being more cautious in terms of spending, and that includes digital ads.

"At $70 [billion], [Facebook] was 50% of global display ad revenue in 2019, so cutting FB ads is the fastest way to cut costs for its clients," Needham wrote in a recent research report. "FB had 7 [million] active advertisers in 4Q19, suggesting that many are small and may have to eliminate ad spending to survive."

The Wall Street firm does think the virus will be contained by the middle of June.

That's the case with e-commerce giant Amazon.com (NASDAQ:AMZN). Last week Tinuiti, a marketing agency that controls around $1.5 billion in annual spending for several advertisers said Amazon has significantly slashed its spend on Google. Amazon is just one customer. If more follow, it's easy to see how ad sales decline. That's particularly worrisome for Google given that part of its business is already experiencing a slowdown in growth. It doesn't help that a big portion of Google and Facebook's ad revenue comes from travel, retail, consumer packaged goods, and entertainment.

There's light on the other side 

Concerns about the impact coronavirus will have on ad spending is already reflected in the share prices of Facebook and Google, which have been plummeting along with the rest of the market all month. Year to date, Facebook's stock is down more than 28% while Google is down 21%.

But there is potentially good news out of the coronavirus-induced malaise. The pullback in advertising should be temporary even if there doesn't seem to be an end in sight right now. Unlike with the aftermath of 9/11, which ushered in a recession, there aren't any underlying weaknesses in fundamentals. Once the virus is contained, assuming it's sooner rather than later, Facebook and Google's ad business should go back to business as usual. Add depressed stock prices to the mix and the loss in digital ad spending presents an opportunity for bargain-hunting investors.