Just have Facebook add a "Download as JPEG" button and let us all go back to more important things.
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.
I'm skeptical of this solution for other reasons, though (like that it assumes users know what JPG means).
For now, yes, but I doubt that's their long-term plan here.