Not surprising.
All the post-iPhone smart phones have blown up the amount of data users take in while at the same time they killed the additionaly revenue streams carriers used to get. (Like charging extra for GPS maps and stuff like that.)
In addition, I think something like less than 20% of Americans have Smartphones. So over the next few years usage is going to explode as more users buy smartphones and those smartphones start to do more and more.
So like I said...not surprising.