Facebook CEO Mark Zuckerberg says 2020 is a critical year for testing the company’s defenses against election interference, and expects to continue to draw heat over how it navigates the choppy political landscape.

Zuckerberg spoke to analysts after reporting fourth-quarter and full-year 2019 earnings Wednesday. Facebook's ad revenue and profits were in line with expectations, but the stock still took a hit in after-hours trading, as some investors wondered if the company has peaked. Facebook generated $21.1 billion in revenue in the fourth quarter, an increase of 25 percent year-over-year, which beat Wall Street estimates of $20.9 billion.

Zuckerberg said “election integrity” is a priority, but also a risk. While Facebook works with law enforcement officials in the U.S. to root out interference from Russia, China, Iran and elsewhere, he said the threat is not just interference. Chaos could be caused with a well-timed meme, which could “sow doubt” about the legitimacy of elections, Zuckerberg said.

Facebook has taken heat over the spread of disinformation on its ad platform, but is reluctant to police speech from legitimate candidates and elected officials. “There is still going to be debate about what kind of political speech should be allowed,” said Zuckerberg, who doesn’t think private companies are responsible for determining boundaries of acceptable political discourse.

Facebook also warned that new privacy measures in the digital ad world could impact revenue growth. “We will continue to face those headwinds,” David Wehner, Facebook’s chief financial officer, said in the call with analysts. “And they will be more impactful as we move forward.”

Facebook has responded to scrutiny by developing tools like the “clear history” setting released this week. It allows consumers to delete their internet history that brands use to target ads on Facebook. If consumers embrace the tool, it could drive down the value of ads, because they would theoretically be less effective.

Google and Apple also implemented new privacy protections that give consumers more control over sharing data.

Facebook executives also warned that ad revenue growth is being squeezed as more advertisers move into Stories, the vertical video format that is especially popular on Instagram. Stories ads generate less revenue than the typical ad in News Feed, according to Facebook.

However, Facebook says it is still seeing an increase in user activity in the main Facebook app as it introduces new services that give consumers new outlets for spending time. Facebook is focused on the video service Watch, gaming, Groups and shopping as new core features.

Facebook spent $15.7 billion on capital expenses—in part for developing new content and protective measures—in 2019, up from $13.9 billion in 2018. It also spent $9.9 billion in marketing and sales in 2019, up from $7.8 billion in 2018. 

Some of that money has gone to an advertising push—a major campaign on TV and other channels—and Facebook is doing its first Super Bowl commercial, too.

The company has warned for several quarters that sustaining the same rate of growth will be more difficult in the future. The company’s trajectory is limited by the number of world internet users, most of whom already have an account on Facebook or its WhatsApp, Instagram or Messenger properties. That means finding future revenue streams will be increasingly difficult, requiring experimentation with avenues that might not pay off, such as in artificial intelligence, virtual reality and shopping.

What Facebook “has to grapple with is a rising cost framework while each incremental dollar of revenue growth gets tougher,” said James Cakmak, a partner at Clockwise Capital.

—with Bloomberg News