Facebook is currently testing a lite version of its app for the iPhone. Facebook Lite is already available for Android users. This app is designed for people who want to minimize their data usage. Facebook Lite for iPhone is currently under the works and is expected to launch soon.
The app is just 5MB in size. So it needs less data to download and less storage capacity on the phone. Facebook Lite for iPhone is designed to use significantly less data and consume less power.
Facebook Lite for Android was launched in 2015, and is also currently in testing for the US population. You need to apply to test it, with a Facebook group established for feedback. Facebook Lite is available only in selected regions as of now and will roll worldwide soon.