What are some alternatives?
When comparing Socket.io and Pusher, you can also consider the following products
Firebase - Firebase is a cloud service designed to power real-time, collaborative applications for mobile and web.
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.
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.
Ably Realtime - Simply better realtime.
Scaledrone - Realtime messaging service