With such a vast shift away from desktop downloadable applications, I'm curious to see what you've done from a marketing standpoint to get those numbers. How did you promote your application and drive sales?
With regards to marketing, see my blog (www.kalzumeus.com), but the short answer is I use organic SEO and AdWords. My one trick pony for organic SEO is scalable content generation, and what a beautiful, loyal pony it is.