That’s a huge drop. For comparison here’s what iMessage did to our family’s text message usage:

It’s important to note that I was using iOS 5 since July, but that not many others were that I would normally text with. Also as you can see we didn’t have nearly as dramatic of a drop off as others had, yet we do text with a ton of iPhone users.

I noticed that in late November and early December a lot more blue bubbles started to pop-up. That is, I believe, people are just now moving onto iOS 5 — these text numbers should keep dropping.