Apparently Facebook has been experimenting with sending WebP images in place of JPEG to some users... but there are some usability problems reported in the tech press < http://arstechnica.com/information-technology/2013/04/chicken-meets-egg-with...
Primary problem seems to be that folks saving images locally or sharing direct URLs get surprised when they don't work in other peoples' browsers or can't be opened in other programs.
Not sure what's a good solution for this, other than a really good download/sharing UI on images...
-- brion