And I will point you to this, about how much data Siri actually uses, and this, how the iPhone manges backgrounded apps.

I highly doubt my mom even remebers how to access Siri; my dad probably uses it more than either of us, and his data is currently sitting at 54mbs, halfway though the month, just like the last 3 months.

As far as I know, Facebook does not run in the background; any notificiations are sent via push, and are not pulled via background checks.

From reading though most of this thread, I suppect it's either a problem with AT&T, or iCloud, but then again, we all three of us have the exact same iCloud settings turn on.