Mike Isaac and Sheera Frenkel

In four new studies, researchers found complicated results from experiments on Facebook’s and Instagram’s algorithms, suggesting there was no silver bullet to fixing the platforms.

In an internal meeting this week, Mr. Zuckerberg said the tech giant was facing one of the “worst downturns that we’ve seen in recent history.”

In an internal meeting this week, Mr. Zuckerberg said the tech giant was facing one of the “worst downturns that we’ve seen in recent history.”