What are some alternatives?
When comparing Socket.io and NATS, you can also consider the following products
Firebase - Firebase is a cloud service designed to power real-time, collaborative applications for mobile and web.
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
SignalR - SignalR is a server-side software system designed for writing scalable Internet applications, notably web servers.
PubNub - PubNub is a real-time messaging system for web and mobile apps that can handle API for all platforms and push messages to any device anywhere in the world in a fraction of a second without having to worry about proxies, firewalls or mobile drop-offs.
Ably.io - Serious realtime infrastructure
Apache Kafka - Apache Kafka is an open-source message broker project developed by the Apache Software Foundation written in Scala.