According to www.nbcbayarea.com, a machine at the company's mailing facility alerted employees on Monday that a package may contain sarin.
Fire officials initially reported that at least two Facebook employees may have been possibly exposed.
"We evacuated four buildings and are conducting a thorough investigation in coordination with local authorities," Anthony Harrison, a Facebook spokesperson told The Verge.
"As of now, three buildings have been cleared for repopulation. The safety of our employees is our top priority and we will share additional information when it is available," he was quoted as saying.
The package was delivered to Facebook and the company began evacuating some of the Menlo Park campus buildings.
Emergency hazmat crews immediately took over the place, asking employees to leave the offices.
Initially, four buildings were evacuated, "and then three of them were deemed safe for repopulation".
"The safety of our employees is our top priority and we will share additional information when it is available," said Facebook in a statement.
Originally developed in 1938 in Germany as a pesticide, sarin is defined as a "human-made chemical warfare agent classified as a nerve agent."
A colourless, odourless liquid, it is used as a chemical weapon due to its extreme potency as a nerve agent.
Exposure is lethal even at very low concentrations, where death can occur within one to 10 minutes after direct inhalation of a lethal dose, owing to suffocation from lung muscle paralysis, unless antidotes are quickly administered.
Sarin is generally considered a weapon of mass destruction. Production and stockpiling of sarin was outlawed as of April 1997 by the Chemical Weapons Convention of 1993, and it is classified as a Schedule 1 substance.
( With inputs from IANS )