What are some alternatives?
When comparing twitter.github.io Kestrel and PubNub, you can also consider the following products
nginx - A high performance free open source web server powering busiest sites on the Internet.
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.
RabbitMQ - RabbitMQ is an open source message broker software.
Socket.io - Realtime application framework (Node.JS server)