うそつきの顔

A yellow face with raised eyebrows, enlarged eyes, slight frown, and long nose, indicating it’s telling a lie in the manner of Pinocchio. Like that character, Facebook's design features a wooden nose. 

May represent lying, a liar, and other concepts of deceit and dishonesty to varying degrees of intensity. Sometimes used to convey disbelief (e.g., You must be kidding) or feeling abashed (e.g., as if caught in the act of lying).