We do indeed measure improvements against more than one website. We wouldn’t want to improve website X while regressing the rest of the internet. See https://v8.dev/blog/real-world-performance (from 2016):

> We now monitor changes against a test suite of approximately 25 > websites in order to guide V8 optimization. In addition to the > aforementioned websites and others from the Alexa Top 100, we selected > sites which were implemented using common frameworks (React, Polymer, > Angular, Ember, and more), sites from a variety of different geographic > locales, and sites or libraries whose development teams have > collaborated with us, such as Wikipedia, Reddit, Twitter, and webpack. > We believe these 25 sites are representative of the web at large and > that performance improvements to these sites will be directly reflected > in similar speedups for sites being written today by JavaScript > developers.

They state specifically in the article that their chart is based on measurements taken from "several popular websites."

Obviously they would want to test Facebook because a lot of people use that website. I'm sure they tested many others as well.

Well.. Facebook runs like a dog, so any improvement is great news.

I consider dogs to be pretty fast :)

Some dogs. Most dogs even. Not my dog. He is a good boy though.

They should start testing against Gmail then :/