When we first noticed Facebook's app was taking up a lot of room on your iPhone, the company told us they were working on a fix. While that fix has arrived, it hasn't really solved the problem.
In a statement, Facebook told us:
"Like many apps, we try to balance how much data we cache on the phone and how much data we fetch over the network. The more we can cache on the phone, the less data we need to fetch over the network. When we save posts, profile pictures, and photos on the device, we don’t need to re-download them when you look at that same photo or post a day later, which both saves data on your network plan and makes the app much faster.
We’re working hard to get better at this balance with every release. Photos and videos account for most of the data we store, and we made a recent update to get smarter about when to delete old photos from the device that aren’t being used anymore. We also made an update in v18 of our app to clear out cached web pages after the web page cache reaches 25 megabytes."