I knew Facebook was a battery hog, but this is over the top. Recently, I have told several friends and family to turn off background refresh for Facebook. It was well known that Facebook drained battery life because it was constantly collecting data and caching the news feed for the next time you launched the app.
Essentially, they are playing silent audio on your phone as a loop hole that allows the app to keep churning even if you turn off background refresh. I'd love to know how they justify that, but maybe people will just keep quiet since we live in a culture dominated by Facebook.
If you want to get back some of that battery life, I would highly recommend downloading their Paper app instead of the default Facebook app. They do not use these tactics in that app (at least not yet :)).