Also used to describe a chic restaurant or bar frequented by "beautiful people" (esp. media/fashion/art types), when it is often shortened to a single "mwah"
"How was it?"
"Totally mwah mwah!"
used either as an oral greeting or in writing (chat, email, letter)
It is usually associated with chick type of thing but not in all of the cases.
Think Alicia Sylverstone in CLUELESS, WHITE CHICKS, LEGALLY BLONDE