What are some alternatives?
When comparing Socket.io and Firebase, you can also consider the following products
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
Android Studio - Android development environment based on IntelliJ IDEA
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.
Xcode - Xcode 4 is the new version of Apple’s powerful integrated development environment for creating great apps for Mac, iPhone, and iPad. Xcode 4 includes the Xcode IDE, instruments, iOS Simulator, and the latest Mac OS X and iOS SDKs.
SignalR - SignalR is a server-side software system designed for writing scalable Internet applications, notably web servers.
NATS - NATS Server is an open source messaging system for cloud native applications, IoT messaging, and microservices architectures.