My pal Greg Scown passed this along. Written by a former Apple Genius:

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.
I have confirmed this behavior on multiple iPhones with the same result: percentage points actually increase after disabling these background functions of Facebook.
Bad, Facebook, bad.

Various things on the list I knew, but I definitely learned a lot reading this.