XMPP

The Extensible Messaging and Presence Protocol (XMPP) is an open technology for real-time communication, which powers a wide range of applications including instant messaging, presence, multi-party chat, voice and video calls, collaboration, lightweight middleware, content syndication, and generalized routing of XML data. The technology pages provide more information about the various XMPP “building blocks”. Several books about Jabber/XMPP technologies are available, as well.

Refer: http://xmpp.org/about-xmpp/

Google introduced Google Talk, a combination VoIP and IM system that uses XMPP for instant messaging.

The later Google dropped XMPP support in Google Voice on May 15, 2014

In February 2010, Facebook opened up its chat feature to third-party applications via XMPP.