What are some alternatives?
Ably Realtime - Simply better realtime.
PushCrew - Push Notifications for Websites
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.
Urban Airship - Mobile marketing solutions across all major mobile platforms.
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
iZooto - Engage your mobile and desktop web users with intelligent web push notifications.