What are some alternatives?
When comparing Socket.io and PubNub, 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.
NATS - NATS Server is an open source messaging system for cloud native applications, IoT messaging, and microservices architectures.
OneSignal - Customer engagement platform used by 800K+ developers; the fastest and most reliable way to send push notifications.
Ably Realtime - Simply better realtime.