The following article was posted by a German developer and shows how the Facebook app consumes CPU time by running processes at regular intervals, going into and out of sleep modes. Since these processes do not take longer than 10 minutes, the app is not constrained by the iOS multitasking governor.
In summary, the only way to prevent the app from using CPU cycles while open is to close the app via the multitasking switcher. Since reading this article, I have been diligent to close the Facebook app after each use. I have been keeping an eye on the battery life but will need some more time to see how much of a difference it makes on battery life not having the app open.
Check out this very informative (and technical) article here.