Right now it doesn't cost much to host. But soon I plan to create a proper plugin store (currently there are only a few plugins available to general public as you may see) and encourage people to develop and upload their own plugins, so I will have to scale up for all that. Regarding the backend, it's a node.js application.
Exactly! socket.io was there mostly to speed up prototyping. I plan to replace it with vanilla code soon, to make the frontend app even more lightweight.