When Google launched the new compact menus in the navigation bar, many users wondered why it's so difficult to sign out. The "sign out" link was buried in one of the menus and it wasn't obvious how to find it. Now the link is also displayed next to the Account menu, where users expected to see it.
Even if it's not used that frequently, the familiar "sign out" link is displayed by most web applications and hiding it or burying it in a menu could frustrate users. Facebook is one of the services that places the "logout" link in a menu, so it's not impossible to convince users to accept this change.