As American security agencies increasingly take to Twitter, Facebook and other social networking sites in search of potential threats, two European travelers say the system lacks one important quality: a sense of humor.

The travelers, Leigh Van Bryan and Emily Bunting, said they were detained overnight after arriving in Los Angeles International Airport last Monday, questioned by agents from the Department of Homeland Security, and then sent back on a return flight to Europe. All because Mr. Bryan joked on Twitter that he was going to “destroy America” during his trip — an apparent reference to partying — as well as dig up the grave of Marylin Monroe — a joke.

“The Homeland Security agents were treating me like some kind of terrorist,” Mr. Bryan, a 26-year-old Irish citizen, said in an interview published in the British tabloid The Daily Mail.

“‘You’ve really messed up with that tweet, boy,’” Mr. Bryan remembered one agent saying, using a more profane expression for messed up.

Some people wouldn't recognize irony if it landed on their heads.