What are some alternatives?
When comparing Heroku and Socket.io, you can also consider the following products
Microsoft Azure - Windows Azure and SQL Azure enable you to build, host and scale applications in Microsoft datacenters.
Firebase - Firebase is a cloud service designed to power real-time, collaborative applications for mobile and web.
DigitalOcean - Simplifying cloud hosting. Deploy an SSD cloud server in 55 seconds.
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
Google App Engine - A powerful platform to build web and mobile apps that scale automatically.
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.