It should come as no surprise - at least no one who's been reading PR/advertising blogs for the last six months - that celebrity endorsers and spokespeople no longer are as effective as they once were. That's the lesson that The Gap learned. A long string of celebrities appearing in their commercials and ads has gone hand in hand with a slipping in same-store sales. There are, to me, three factors at play here:

1. Celebrities are under more scrutiny now than ever before. A wide variety of sensationalistic magazines and TV shows mean that the private lives of stars are aired for all to see constantly. It's very hard to assign credibility to someone you know has cheated on their wife/husband, whether it's a celebrity or your neighbor. They chose to focus on celebrity endorsers because it worked in the past. So did aquaducts but then someone discovered you could put the pipes below ground. Adapt.
2. Blogs and other social media have built a network of trust and reliability for their unfiltered and unadulturated opinions. Why should someone believe Sarah Jessica Parker - when she was paid to endorse a product - when an ordinary citizen and consumer gives you a more open and honest opinion.
3. As the article states, Gap's brand name doesn't stand for anything anymore. There are too many other stores in the mall, all angling for the same group of 13-25 year olds. The Gap may have been where your older sister shopped but she's, like, totally out of college already.