T-Mobile customers use the most data, AT&T comes last
As you can see from the graphs below, T-Mobile subscribers are still the absolute leaders in monthly data usage among all four major US carriers, with about 5GB on average, followed by Verizon at about 4GB, Sprint, and, finally, AT&T with the extremely modest for today's standards less than 3GB. This is for cellular data, but Wi-fi usage distribution gives us a similar picture, with Sprint and T-Mobile followed by Verizon this time, and then AT&T. Ma Bell ran a promo in Q4 that brought in a lot of prepaid subscribers on bring-your-own-device plans that have lower data limits, which could explain while AT&T is last in average data usage.
When broken down by plan type, the users on unlimited are using more than twice the average amount of data than those with monthly quotas, while the pay-as-you-go crowd are only a fraction of the consumption. Given that "data prioritization" aka throttling kicks in at much higher thresholds on all carriers, especially T-Mobile, those average numbers look pretty modest to us anyway, but let's not forget that they include all subscribers, even those without data plans, or on prepaid with cap limitations.