No, they didn't. The web works fine. They can browse it without any issues. The problems occur when they're trying to do stuff on their local machines.
> Not for users, no. The only gain here is facebook's bandwidth costs, at the expense (and inconvenience) of its users.
Change rarely occurs smoothly.
By all means, try to explain that to users if you like, and if you think they'd care. The issue is that it breaks the user experience, whatever technical reason you'd like to attribute that to.
The users are completely irrelevant, because no matter how much they bitch and moan, they're not going to leave Facebook. Its network effects are too powerful. And so, for once, I actually support Facebook. They're using their dominant market position to push a new standard that is quite good. This will hopefully force companies like Adobe to start adding support for WebP by default. Then we'll finally able to ditch obsolete standards like JPEG and its ilk once and for all.
Facebook is still doing what Facebook intends to do, in fact they do it better! It's unfortunate that a tiny margin of users are slightly inconvenienced, but they need to get over it.