Servers are on AWS and Cloudfront. We started out with three.js for rendering but we froze at 78 and are currently working on rewriting the renderer to not use the scene-graph and instead move to a more data oriented design.
We did look at Rust, but Rust suffers from a little bit of the "shiny new thing" syndrome. We have simple needs and wanted something fast and boring :)
The server-side design is also as boring as possible, just standard web stack for serving, and websockets for multiplayer. We use EC2 for everything yep!