It won't let you delete it by organising, and it won't let you delete it by App Manager > Installed Apps > Long Press > Unistall it just says 'Removing Cancelled' in both cases. I've removed the facebook installer but this makes no difference.
Some of the preinstalled applications can not be removed from the phone. That is why you get the 'Removing Cancelled' message.
If you are trying to delete the app to free some space, try deleting some other apps or files. Clearing the cache will also free some space: Menu > Web > Options (right bottom corner) > Settings > Privacy > Clear pricvacy data.