Rather than dwelling on the phrase "they broke the web", read it as "they broke an important use case".

Facebook is integral to the everyday lives of many ordinary people, and the lesson here is that the designers at Facebook need to think beyond the specific tech and content that they build and deliver; they need to also consider the context in which their product is used.

What about the context of "my page is loading too slowly" or "Facebook is using all of the data in my monthly plan"?

Some users are facing a temporary inconvenience but everyone gets an immediate & long term gain.

It ought to go without saying that Facebook is trying out this new image format because they expect benefits.

Not everyone: only people who are bandwidth constrained.

I imagine the use case on the Facebook side is "viewing the image on Facebook" rather than anything beyond that.