Unlimited app data applies to data usage within the specified app.
For example if you purchase an offer that includes Facebook, then any data consumed within the Facebook app will be sponsored. This includes when you read an article that someone has shared via Facebook, as long as you are accessing the data within the facebook app the data will be unlimited consistent with the purchase offer.
However if you open a mobile browser (e.g. Chrome, Safari, etc) and load facebook.com or an article that was shared via facebook, data will not be unlimited.