Shares of Facebook fell 5.2% Friday at the opening bell, as Wall Street reacted negatively to big changes CEO Mark Zuckerberg outlined for the social service’s News Feed.

In a post Thursday evening, Zuckerberg said the social giant in 2018 will re-engineer News Feed to emphasize content from friends and family — rather than posts and videos from media publishers, businesses and brands — in the year ahead.

“One of our big focus areas for 2018 is making sure the time we all spend on Facebook is time well spent,” the Facebook chief wrote.

Zuckerberg said the shift also will mean that “the time people spend on Facebook and some measures of engagement will go down.” The company said the News Feed changes will be rolling out over the next few months.

In response, Facebook’s stock saw its biggest decline in over a year — as investors fear the News Feed changes could crimp the company’s growth and ad revenue, at least in the next few quarters. The stock opened Friday at $178.06 per share — down 5.2% from Thursday’s closing price of $187.77. [UPDATE: Shares were down about 4% at 11:05 a.m. ET on heavier-than-usual trading volume. The stock closed the day down 4.5% at $179.37 per share, representing a $24 billion drop in Facebook’s market capitalization.]

“We think the actions the company will take pose a headwind to growth for the business in the near-term,” Pivotal Research analyst Brian Wieser wrote in a research note late Thursday. “However, we also think that the actions taken will be helpful for the long-term health of the overall business.” He maintained a “sell” rating on Facebook with a year-end 2018 price target of $147 per share.

Facebook has routinely topped Wall Street forecasts for quarterly revenue and earnings. The concern for investors is that if the News Feed modifications reduce time spent on the platform, ad revenue also will take a hit. Adult Facebook users in the U.S. were on track to spend an average of 41 minutes per day on the platform, versus 40 minutes in 2016, according to estimates by research firm eMarketer.

While Facebook’s ad load could drop as a result of the News Feed update, the company may raise pricing to offset that, RBC Capital Markets’ Mark Mahaney wrote in a research note. “In our view, making the feed more relevant should boost user and engagement growth over time,” he wrote. “Facebook is making the service more social and less media, and that’s likely a positive for the vast majority of users.”

Zuckerberg, in his post, said Facebook users can expect “to see more from your friends, family and groups” in their News Feed. In addition to surfacing fewer posts from businesses and publishers, he added, the revamped News Feed algorithms will favor third-party content that meets the standard of encouraging “meaningful interactions between people.”

“By focusing on bringing people closer together — whether it’s with family and friends, or around important moments in the world — we can help make sure that Facebook is time well spent,” Zuckerberg wrote.

Facebook noted that users can set their News Feed preferences to make sure they always see posts from their favorite Pages.

Adam Mosseri, Facebook’s head of News Feed, wrote in a separate blog post that as a general rule, “Page posts that generate conversation between people will show higher in News Feed.” For example, Facebook Live videos on average get six times as many interactions as regular videos, according to Mosseri. He also said that many creators and celebrities who post videos on Facebook prompt discussion among their followers, while “local businesses connect with their communities by posting relevant updates and creating events.”

Using “engagement-bait” tactics designed to “goad people into commenting on posts” do not not represent meaningful interactions, Mosseri added, and “we will continue to demote these posts in News Feed.” Facebook has similarly cracked down on clickbait headlines in the past.