A graph presented at
Compete shows the sites where people spend the most time. Unfortunately, it doesn't have a breakdown that shows where people spend their mobile time, but I'm sure one will eventually pop up.
MySpace is the champion. I wonder how long they will be able to hold that spot. And note that Google is only #5, but that's because the graph doesn't represent how often you use a site, but how much time spent there.
The other obvious anomoly is the absence of MobileRead.com. Surely it was a sampling error! ;-)