Not really an answer but the creator of agar.io described succinctly his stack : server in C++ with libuv for the networking.
- NodeJS for web server - C++ for game server - Redis for data storage - Nginx + Varnish for caching and routing - Websockets for client <-> server communication - UglifyJS for source code compression - Planet icons designed by Freepik (http://www.freepik.com)