WASHINGTON (AP) — The U.S. Secret Service is investigating an online survey that asked whether people thought President Barack Obama should be assassinated, officials said Monday.The poll, posted Saturday on Facebook, was taken off the popular social networking site quickly after company officials were alerted to its existence. But, like any threat against the president, Secret Service agents are taking no chances.”We are aware of it and we will take the appropriate investigative steps,” said Darrin Blackford, a Secret Service spokesman. “We take of these things seriously.”