Lots of people treat backbone private lines as data when it could be carrying voice. I really don't think anybody really knows the relative size of these two traffic flows. Most T1s were deployed to avoid switched access charges, not to carry data traffic. That's if all ports are busy all the time. Yes, but link capacity is a poor measure of traffic.
Data traffic is just too bursty.
You might want to take a look at the AboveNet website. I believe they have some utilization stats. Might be insightful. That's where I get bogged down. But capacity utilization on data networks appears to be extremely low.