Many iOS users have been reporting excessive battery drain (up to 15% of all battery usage) when using Facebook on their phones. The issue, it seems, is Facebook continuing to run processes even when background app refresh is disabled.
Facebook has commented on the issue, saying, "We have heard reports of some people experiencing battery issues with our iOS app. We're looking into this and hope to have a fix in place soon."
They have declined to comment on exactly what's causing it or why, leaving the media to speculate. One theory points the finger at Facebook's auto-play video feature, but testing on that is inconclusive.