1. Use Chrome for web browsing
If your smartphone of choice is an iPhone, you should download another browser besides Safari -- we suggest Chrome -- for your mobile web surfing. Chrome doesn't simply have Google's reputation backing it, it also lets you alter your settings to compress web pages for mobile browsing. Just tap the three dots in the upper right-hand corner of the screen, scroll down to Settings > Bandwidth, and turn on Data Saver.
Another option is to use a browser specifically designed with data-saving in mind, such as Opera Mini.
2. Turn off Facebook's auto-play videos
All those videos of babies and puppies and babies riding puppies that auto-play in your Facebook feed? They eat up a lot of data. In Android, turn off auto-play by opening Facebook, going to Settings, and switching "Videos auto-play" to Off or Wi-Fi Only. In iOS, go to Settings, select Facebook, Settings, then switch Auto-play to On or Off under Video.