Is there any way to reduce an app's usage of storage space on iOS 5?
You could delete the Facebook app and then reinstall it. When you delete an app like Facebook, there's a little warning saying that deleting this app "will also delete all of its data", which is exactly what you want.

I just tried this on my iPhone - deleting the Facebook app and reinstalling it made its usage go from 47 MB to 16 MB. This is only a temporary solution of course, since as you use Facebook it'll cache images etc. to speed up your browsing, so it'll start using more space again.