I have been working on a small application with the goal to gather
common use cases around the ActivityStreams specification. I found
that in the discussions here, and in the specification itself, the use
cases are not always easy to follow, and maybe this application can
help in the future:

http://streamaweb.info/

It's only a 'post' verb with some basic 'network_update' object
implemented right now, but I hope to get some feedback of you, what
verbs and/or objects would be important to work on next. Also, I plan
to cover a following of streams.
The application is build around the Streama gem in Ruby, and other
open-source tools, and I am thinking to keep the application
open-source. However, as the use cases might have business value,
there might be business interests coming out of the application too.

Thanks for your interest, and feedback eventually.

Best regards,

Patrick