We're really excited about shared streams - so much so that we have been discussing how we have been using them so far.
Here are some of the teams ideas and use cases:
Document team processes, especially if they evolve often or are new
List content resources that people can contribute to and passively check when they want to read something new.
Company library detailing the books we have and who currently has them
Public to-do list for leaders that are super transparency-focused
Share a private note/project when it is ready for feedback
Ad-hoc release notes/updates
Discuss and collaborate on projects outside of meetings
Sales + CS:
Passing action items back and forth for customer-related work
Collaborate outside of calendar event to leave more calendar real estate for training/demos
Process Checklist (to publish a blog post, run a webinar)
Release notes (and can have a template with a deploy checklist).
Document processes like (how to make a pull request, how to hand off a feature, how to request time off…)
Feel free to let us know how you're using them - we're always intrigued by interesting use cases and "hacks"