The fact that the iPhone has a 5% market share does not mean it represents 5% of all SMS messages sent. Smartphone users text message more frequently than your grandmother with an LG handset.
I would argue that the frequency of SMS usage has very little to do with the type of phone you have, and much more to do with the demographic. There are plenty of iPhone users (such as my mother) who don't use SMS at all, and plenty of 14 year olds without smart phones (like my cousin) who text dozens of times a day.
Either way, I've never seen a breakdown of SMS usage by either category.
I had peers in junior high/high school (before I even had a cell phone) who sent hundreds a day. One girl bragged about reaching something like 30,000 one month.
