The new width shouldn’t surprise anyone, Facebook having announced the move in October 2009. The social network simple took their time to roll out the new width so administrators could get ready for the new page tab width.
According to Inside Facebook other parts of the site, such as popular Pages will continue to use 760 pixels.
Page owners will want to change any elements on their pages to 520 pixels as soon as possible to avoid awkward looking pages.
Take a look at page tabs now to see the new look when compared to other page width options for non-tabbed pages.