I've never bought from Scribd, but I do buy from Smashwords (especially those authors that are on MR).
Just because a book doesn't have as many downloads from Smashwords as Kindle doesn't mean that it isn't being seen or purchased. (The book is still getting exposure, if you qualify for the premium catalog you are available through other stores... from their info page
3. Premium Feed: This new catalog is distributed to major online retailers such as Barnes & Noble, Sony, Kobo (formerly known as Shortcovers) and Apple, and other distribution outlets. These retailers have higher mechanical standards such as requiring quality book cover images, books with copyright pages, and other simple requirements outlined below. If you're a serious author or publisher, you want your books included in Smashwords Premium Catalog. For special requirements of the Apple catalog, see this checklist, How to Publish Ebooks on the iPad with Smashwords.
I have the Sony 505 and friends with Sony's and Kindles. My friends with Kindles will look for books on Smashwords then usually look for the same book on Amazon and buy it there. Their reasoning is so they can keep all their books in one place plus whispernet it to their Kindle. They discover and try indie authors because they saw them on or through Smashwords.
EDIT: forgot to mention that if the book isn't on Amazon they will buy it from smashwords.