Data usage

Hi all,

i just got the Iphone 5 two weeks ago and got a second line for my brother,he has the 4.I only have 6GB to share between the two of us and im use Verizon.After 3 days of using the phone my data was already 50% used.I sign out of facebook,twitter or any other apps that might be using my data.I also use my wifi at home and turn off cellular data.What can be using the data?

Please let me know because verizon couldn't answer any of these