The Facebook app on iOS once again became a cause for concern after users started noticing the app drawing considerably more power and draining the battery. Facebook has now fixed the issue in the latest update, which was pushed out yesterday.
Although Facebook didn't bother pointing out the reason to fix or even acknowledge the issue existed in the update log, Facebook engineering manager Ari Grant wrote in a Facebook post the two reasons causing the battery drain.
The first cause is what Facebook calls CPU Spin bug in the network code that kept waking the CPU. The second one is where the app kept the audio session running in the background after a user watched a video in the app and then closed it. This is usually used by music players to keep the audio running but shouldn't be the case with Facebook. It prevented the app from idling after a while and kept running in the background.
Questions have been raised regarding the legitimacy of Facebook's "bug", as some think Facebook was trying to get around Apple's restriction of allowing apps to run in the background consistently using workarounds and was eventually caught, so had to resort to calling it a bug. Whatever your stance is, the issue has been fixed now.