Washington Post Staff Writer Sunday, October 29, 2006 Teen Web sensation MySpace became so big so fast, News Corp. spent $580 million last year to buy it. Then Google Inc. struck a $900 million deal, primarily to advertise with it. But now Jackie Birnbaum and her fellow English classmates at Falls Church High School say they're over MySpace. "I think it's definitely going down -- a lot of my frien