Facebook's California office was placed on "lockdown" after an apparent security threat, police have said.
The office at Menlo Park in San Francisco was searched by officers after receiving an unspecified threat.
Staff were held until 8.30 PM while police made sure the area was safe.
But eventually the threat was dismissed as "not credible", according to Sky News.
Menlo Park Police Department spokesman Dave Bertini told reporters "it was a totally not credible, unsubstantiated threat".
"I'm not even sure that it was specifically to the Menlo Park campus of Facebook," he added.
Facebook has not commented on the incident, which ones a week after the news that Mark Zuckerberg's social network would pay for a full-time police officer to be based at its offices.
The company will pay $200,000 for three years to keep the officer at the site.