I am in the same boat! My data sky rocketed after I upgraded from IPhone 4 to 6. I checked the data utilization and it says most of my data is being used by social networking??? The only thing I have is Instagram and I'm not using it any more than I did with my 4. Hope someone can help!
Remember the 6 has 10x faster internet with LTE than the 3G iphone 4. That means more will download before you switch and video will come in higher def, also causing more data. A study showed going from 3G tech to 4G tech, while "doing the same things" uses 2 to 2.5 times more data than before. This is across carriers and not just Verizon. You can confirm by stoppinglooking at Instagram for a whole and see if the data use goes down and your social networking data use goes down.