If it doesn't give you a choice, It can't be moved. Also, a widget has to be stored in internal memory. Once moved to phone storage, it looses its function as a widget. I hope someone can further explain this.
Facebook (and any other preinstalled app) is cooked into the ROM, so it can't be moved to the SD card. Only fully installed apps from the Play Store can be moved. This is another advantage of rooting and ROMing, since most custom ROMs remove the preinstalled apps in an effort to redice the size of the ROM. This way every installed app from the Play Sotre can me moved to the SD card. I don't know why manufacturers insist on preinstalling apps that can be manually installed by the user from the Play Store?!