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.
Why would anybody care about the format that Facebook users use? I see way more value in reencoding company website images in WebP so that things load faster at the same quality. This is especially useful if your visitors are fairly technical, i.e. when you are a systems software vendor or when you're a tech blogger: even if WebP is only supported by Chrome and Firefox, it means half of your audience can enjoy the faster loading times. With HTTP content negotiation you can gracefully fallback to JPG. WebP does not need the support of Facebook users to become valuable.
Facebook users, myself included, care a good deal.
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.
Flickr, Picasa and such re-encodes the images to various sizes for display even if the untouched original is available. I believe that many phones offer to downsize before upload to save bandwidth in the other direction too.
