Facebook's app is killing your iPhone's battery, and unless you delete it completely, there's no way to make it stop.
The problem stems from the fact that Facebook manages to run in the background constantly, even when you forcibly close the app on your iPhone. And turning off the app's ability to run in the background doesn't solve the problem either.
How the Facebook app runs in the background, even when you tell it not to
On October 11, a Medium post by Matt Galligan showed the Facebook app accounting for the majority of his iPhone's battery drain by running in the background. The kicker: Galligan had explicitly turned off Facebook's permission to run in the background from his Settings app.
"Over the last 7 days, Facebook is the greatest offender of battery drain on my iPhone 6s Plus," Galligan wrote, noting that the app "accounted for 15% of all battery drain."
You can see which apps on your iPhone are allowed to run in the background by going to Settings > General > Background App Refresh. Apple has this setting in place to prevent apps from violating your privacy and draining your battery when you're not using them.
It's one thing for Facebook to drain your battery in the background when you give it permission to do so.
But it's another thing entirely for the app to run in the background when you've explicitly told it not to.
So how is Facebook doing this? The consensus among people on Reddit affected by the battery drain is that Facebook "abuses" a provision Apple allows for Voice-over-IP (VoIP) apps like Skype to always run in the background so they can do their jobs, like let you answer incoming calls even when an app isn't open.
This behavior could explain why "audio" is displayed as the reason for Facebook's background activity.
Another possibility is that the Facebook app sends out silent audio signal to stay open in the background.
Since the main Facebook app has no VoIP functionality (it can't make voice or video calls over the internet), it would be in direct violation of this clause in Apple's App Store guidelines for developers (emphasis added):
"Multitasking Apps may only use background services for their intended purposes: VoIP, audio playback, location, task completion, local notifications, etc."
Violating Apple's rules is grounds for an app to be removed from the App Store.
The Facebook app for Android is more complicated to diagnose given the many versions of Android there are in the wild, but a recent study by antivirus company AVG found Facebook's app to have the worst "overall performance impact" for Android users.
Why the Facebook app drains your battery constantly
It's unclear why Facebook would intentionally skirt Apple's rules and cause its app to run in the background at all times. Some have suggested that the reason is to deliver faster push notifications and more effectively track the user's location.
"We have heard reports of some people experiencing battery issues with our iOS app," a Facebook spokesperson told Tech Insider. "We’re looking into this and hope to have a fix in place soon."
What you can do about it in the meantime
Delete the Facebook app. Unfortunately, there doesn't appear to be a way to keep Facebook from running in the background and draining battery life on the iPhone.
Besides the mobile version of Facebook's website, iPhone owners do have another option with Paper, an experimental app from Facebook that uses less battery life.