Where are most pictures on the Internet? I would guess Facebook and Flickr, uploaded by users who haven't the skill or the time to bother touching up the pictures they took today.
Facebook's introduction of 2000px wide images (close to the output of most consumer cameras) makes this all the more important. If Facebook's letting people upload directly from their cameras to the site with no scaling, the format the camera uses is a big deal.
Also, Facebook is a very large portion of photos shared over the web.