Some teams may use it at the big companies and I myself have used lesser performant languages at some of the big companies but only for smaller stuff. For the widely used stuff which big companies do a lot of, you need high efficiency and javascript ain't gonna do that.

One of my co workers worked on a 5 person project for half a year that just refactored a core algorithm for a heavily used product and that halved the 5 million dollars a year in hardware costs and that's not expensive public cloud cost but the highly efficient internal data center costs and all of the code was in C++. Writing any of that code in javascript would just be wasting money.

I don't think Amazon, Google, or Facebook do on any significant scale.

Sure. Do you know of any studies that analyze the power consumption of different languages on the backend? I'd be curious to see how they compare.