"Ciao," meaning "goodbye," is fairly common in the United States but it carries a sense of sophistication--as if you were someone rich who traveled to Europe at a lot. "Chau" must be common in Latin America because some of my Spanish language partners use it.
ahdictionary.org includes it as a legitimate U.S. English word, and notes that "The Italian salutation ciao, which is now popular in many parts of the world outside Italy." It says that it "originated in the dialects of northern Italy. In the dialect of Venice, ciau literally means 'servant, slave,' and is also used as a casual greeting, 'I am your servant.'"