When you’re getting ready for the big dance, you slip into something nice, clean and pretty to try and put your best foot forward for the crowd. Two major sites did that today. Facebook doffed the equivalent of a new pair of shoes in anticipation of tomorrow’s F8 conference, drastically changing users’ News Feeds while keeping the rest of the layout the same. Pandora took the opposite route; they overhauled their ensemble from the ground up in a quest to impress. Unfortunately, one of them dashed its prom queen hopes after getting a big FAIL from unhappy users.