Like a smiley face, the double chin smiley face indicates someone is happy but pulling a double chin whilst smiling
It can also be used in a sarcastic manor. Once in a online or text chat, the person uses this face to indicate sarcasm or mock the person speaking to them.
Kyle: Are you going to that party tonight?
Lauren: Er, no
Kyle: How come?
Lauren: Cos i'm not a beg :-))