I get what you are saying, but I have not used my iPhone for anything other than email.
Not based on this copied from your original post.
Using the same apps (fb, email, Linkedin) with no push notifications.
Using the FaceBook app and the Linkedin app uses cellular data when not connected to an avaialble wi-fi network and much more than the same apps with a Blackberry for the reasons already provided. You haven't used Safari for any web browsing, the same for Weather app or the Maps app when not connected to an avaialble wi-fi network?
Have any of the received or sent email message included attachments?
If the iPhone is not connected to a power source when connected to an available wi-fi network, the network will be disconnected while not actively using the iPhone and when putting the iPhone's screen to sleep.