> My social network streams are more and more filled up with multi-megabyte animated GIFs, and WebP is a reasonable replacement for both animated GIFs and transparent PNGs.

It's only slightly better than lossy PNG:

http://pngmini.com/vs-webp/

It doesn't appear to be good at beating GIF, and loses to APNG in compression (and I suspect also decoding speed):

http://littlesvr.ca/apng/gif_apng_webp1.html

and I find it highly questionable whether a new format that is exactly as expensive as WebM intra frames should be used instead of WebM.