Traffic seems to be a very poor metric for comparison in this case.
Every scribd page that I looked at (from Reddit and Digg) could have been done just as well (and in some cases, substantially better) as HTML, using any blogging software (e.g. WordPress, blogger.com) instead of Scribd. The value-added features it offers (basically, a worse-is-better reimplementation of Adobe Acrobat, and collaborative editing) only caused me discomfort.