I <think> the people hub works on push notifications, so the phone doesn't call back to Facebook all the time to check if there's anything new, Facebook sends a notification if there is something new. I also seem to recall that within the Settings section, you can select an account and choose how often it syncs, so you can set this to be 'on request' if you want.
The Lumias have far better battery life than the Wildfire did, mostly just by being newer.
I had a wildfire for two years - always had the background data off to save battery life and also to ensure I wasn't getting updates when out and about or using internet unknowingly.
But just today got a new lumia 900. How do I do the same thing? My notifications seem turned off on fb as I don't get alerts which is fab, but in concerned with the link with "people" with Facebook etc in using data and battery when I don't want to.