Or just have the browser add a "Save Image as JPEG" button that does automatic conversion.

I don't know if jpg can reuse the information from webp or if you have to do full uncompression and then recompression but if you care about that you probably wouldn't have this problem in the first place.

But then Facebook have to store two formats, something I'm sure they don't want to do.

They're doing that anyway because they serve non-webp images to users with non-webp-supporting browsers. Anyhow, this is the image that's for display, which is resized/compressed; they store and allow you to download the original no matter what.

I'm skeptical of this solution for other reasons, though (like that it assumes users know what JPG means).

Users who don't know what JPG means are not going to be attempting to edit the images in photoshop. Although perhaps they are saving the files to their local windows machine.

> They're doing that anyway because they serve non-webp images to users with non-webp-supporting browsers.

For now, yes, but I doubt that's their long-term plan here.

But then the jpg file will be optionally downloaded, thus reducing the bandwith usage (desired effect), and accelerating the page-load (another desired effect).