Scotty Loveless has written the single best post on iOS battery drain:
iOS 7 made it super fun to close your apps: all you have to do is double-click the home button and swipe up on the app preview to blast it into a digital black hole.
What most people tell you is that closing your apps will save your battery life because it keeps the apps from running in the background.
This. Additionally, Facebook seems to be one of the strongest battery sucking app:
During this testing, Facebook kept jumping up on the process list even though I wasn’t using it. So I tried disabling Location Services and Background App Refresh for Facebook, and you’ll never guess what happened: my battery percentage increased. It jumped from 12% to 17%. Crazy. I’ve never seen that happen before on an iPhone. The iPod touch exhibits this behavior, to my memory, although I haven’t tested it in a while. For the iPhone, the battery percentage is usually pretty consistent.
It’s interesting that the iOS battery percentage is actually an estimate.