Police searched and secured the headquarters of social media giant Facebook Inc on Tuesday evening after receiving a threat against the company, which San Francisco-area authorities said turned out to be "totally not credible".Facebook personnel held employees at the campus until officers were on scene, Menlo Park Police Department spokesman Dave Bertini said. Police officers allowed Facebook staff to leave at about 8:30 p.m. (0330 GMT)."It was a totally not credible, unsubstantiated threat," Bertini said, though he declined to say what the specific threat was. "I am not even sure that it was specifically to the Menlo Park campus of Facebook."Facebook declined to comment on the incident.The San Francisco Police Department did not immediately respond to requests for comment.