Jive's Real-Time Chat (RTC) service has been discontinued in the latest Jive Cloud release (2015.2) on June 26, 2015.
Previous chat messages are still accessible through direct messages in your inbox in Jive.
As an alternative, Jive offers Jive Chime for real time chat functionality, as well as integrations with systems like Lync.
If you are interested in these products please reach out to your account manager.
More information about the removal of Real-Time Chat and other changes in Jive Cloud 2015.2 can be reviewed here: Jive-n and Jive-X 2015.2 Upgrade and Onboarding Guide