In December Salesforce notified customers that “Salesforce is retiring Chatter Messenger on March 30, 2018. At that time, you’ll no longer be able to use the feature, and Salesforce will not provide any support for it.”
Chatter Messenger is an pop out client that allowed Salesforce users to chat with people while they browsed other sites and used other applications, complete with buddy list. .Chatter Messenger has not been available in new orgs created after the Spring ’16 release. Salesforce have given plenty of notice.
Salesforce recommend Skype for Business: “We encourage you to migrate to Skype for Salesforce integrates Salesforce and Skype for Business®, so your sales and service reps can send and receive instant messages and see colleagues’ availability without leaving Salesforce.”