Here at AP, we currently use Slack to communicate with and send messages to our colleagues. It's a great tool, and we don't have any real complaints. Prior to our migration to Slack around two years ago, though, we used Hipchat. That Atlassian service, along with Stride, has now been purchased (and shut down) by Slack.
Atlassian decided to exit the chat business because it just wasn't as successful as expected, and it wants to focus on project management software. In exchange for Hipchat Cloud and Stride's IP, the company will be getting a "small, but symbolically important" equity investment in Slack. Slack will also be paying a "nominal" amount to Atlassian over the next three years for the IP. Slack and Atlassian will continue to collaborate via Slack integrations such as Jira Server and Cloud, Trello, Bitbucket, and more.
Those of you who use Hipchat and/or Stride shouldn't panic just yet; Atlassian will continue to manage its products and customers until February, at which point the cloud services will be killed off. However, customers who have Hipchat installed on their own servers will be able to continue using it for anywhere between an additional few months and two years, depending on the software version. Slack will be providing migration procedures and even webinars to get former Hipchat customers on board.