What are some alternatives?
When comparing twitter.github.io Kestrel and Socket.io, you can also consider the following products
RabbitMQ - RabbitMQ is an open source message broker software.
Firebase - Firebase is a cloud service designed to power real-time, collaborative applications for mobile and web.
CloudAMQP - CloudAMQP automates every part of setup, running and scaling of RabbitMQ clusters. Available on all major cloud and application platforms.
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
Amazon SQS - Amazon Simple Queue Service is a fully managed message queuing service.
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.