There's a thread going on about this right now (http://forums.macrumors.com/threads/facebook-app-background-and-battery.1909817/page-2#post-21988827). Bottom line is, if you're having horrible battery life all of the sudden then the culprit is the Facebook app. Facebook recently released a new version (v. 40.0) that is bugged to hell and will run in the background constantly for hours on end. I took a look at my Battery usage after reviewing the thread above and sure enough Facebook was basically using over 50% of my battery with 1.5 hours of on screen usage and 6+ hours of background usage! I have since uninstalled Facebook and now my battery life is great, I'm currently sitting at 55% with 6 hrs of usage and 9 hrs of standby.