Slack is a collaboration tool that's well known among the DevOps and developer communities. It has been widely embraced due to the extensible nature of the app. In fact, it's safe to say that Slack is more of a platform than just a chat tool. Slack supports "bots" which are apps that can do all sorts of things, from answering simple questions, to deploying software.
Slack is used primarily by private organizations, however, government agencies have begun to test the waters. Slack is actively pursuing wider use by government customers. They recently posted a position on popular job board LinkedIn for a "Manager, Government Strategy" to work in Washington, DC. Specifically, the role is to "manage relationships with government accounts and help develop our government program at Slack " The posting can be read at this link (https://www.linkedin.com/jobs/view/manager-government-strategy-at-slack-577290692/). If Slack has their way, more government agencies will adopt it.
Agencies with Slack Channels
1. 18F - 18F is a unit within GSA that often takes the lead when it comes to government agencies using tech popular in the commercial world and adapting it to government use. The rules related to use of Slack by 18F are well documented. The rules are documented here (https://handbook.18f.gov/slack/). 18F also has a handful of public slack channels for discussing such topics as DevOps, eRegulations, and the Open Data Maker project. Access is granted after submitting an application (https://chat.18f.gov./)
2. U.S. Census Bureau - The Census Bureau operates a public slack channel which shares information related to data provided by the agency. The slack channel URL is here (uscensusbureau.slack.com). The conversations here can get fairly technical as they delve into the meatier parts of the apis used to access and retrieve census data. It's definitely a place for programmers.
3. US Department of State - The Bureau of International Information Programs appear to have used slack in at least one case. There is a mention of Slack usage at TechCamp Thailand in 2017 (https://techcamp.america.gov/blog/mealformarawi-techcamp-thailand-inspired-outcome/), The use here appears to have been to help foster communication among the participants in Thailand as it was mentioned alongside Facebook.
4. NASA - NASA's Jet Propulsion Lab not only uses Slack, but is prominently featured as a key user by Slack (https://slack.com/customer-stories/nasajpl). I suspect there are more uses for Slack within NASA given the size of the agency.
These early adopters are paving a path for other agencies to follow. How and where to use Slack are the next questions. The Census Bureau methodology makes a lot of sense. I can see other agencies with public data sets making similar channels. The collaborative nature of Slack combined with bots what can quickly answer frequently asked questions seems inevitable. I could also see Slack adopted at the city level where they too publicly distribute data.
Internal use could be different. Slack makes it easy to share and store a lot of information besides basic chat. If used for ChatOps, then logs of application development activity is stored. But Slack also allows for voice and videoconferencing. Some of the tools record the meetings and trnscribe them. While great for internal team info, this might not be preferable to share with the general public. And then there are documents that can be stored and shared via the tool. It's unclear still what types of information, is subject to FOIA requests. The idea being that government e-mails are subject to FOIA, so Slack should follow a similar path. For public data, that's not an issue, but if other types of data are shared over Slack, there could be issues. The solution here may be for agencies to use a locally hosted Mattermost install which provides similar functionality, but is hosted privately by the agency.