As one of the world's largest advocates for web access in emerging markets, Facebook obviously cares very much just how well its own platform works on mobile devices in those markets. As such, Facebook sent a team of product managers and engineers to various regions in Africa to learn more about just how people were using the service, on which devices, and what the major pain points for the app were.
Their findings will in no way whatsoever shock you: it wasn't fast enough, it used too much mobile data, it failed to load images pretty often, and the app was substantially overweight in terms of storage footprint. Wow!
After realizing that its app was a bloated, unreliable pig (let's not pull any punches here), the mobile team set to work on improving the experience. The results are admirable: 50% reduced startup times, 50% reduction in data use compared to last year's app, 90% fewer image load failures / slowdowns, and a 65% drop in the size of the app. The changes to the app resulting in these improvements are all currently live, having been implemented throughout the year.
Some of the changes are pretty interesting. For example, Facebook has started using the Google-built WebP format for images on mobile, reducing image sizes by 25-35% over JPG, and 80% compared to PNG. Images also no longer load at full-size by default, and only request a full-resolution picture if you use the zoom feature. Optimizations to startup routines for single-core devices have helped speed up the app on older phones, and enhanced caching for the news feed to keep the perceived "loading" times down.
You can read a lot more about Facebook's findings on the official post here.