Facebook Inc evacuated four buildings and two people were being evaluated for possible exposure to the nerve agent sarin on Monday after a package at the social media company's Silicon Valley mail facility tested positive for the toxic compound. The people who came in contact with the suspicious package at about 11 a.m. PT (1800 GMT) did not show any symptoms of exposure to sarin, said Jon Johnston, fire marshal for the city of Menlo Park, California, where Facebook is based. "The (Facebook) facility tests all of the packages that come in and they had a positive test, so ...