I wouldn't say that Piazza reduces faculty workload, but it does improve teaching. When a student is facing a project deadline and is stuck, that provides a teaching opportunity where they are paying attention. I am often answering questions on Piazza late at night when I and the students are not on campus, so it provides remote connectivity.
Piazza has proven to be a valuable tool. I answer questions one time and the entire class has access to those answers.
Piazza is working extremely well for me. The discussions are much tighter than what I used to see on email lists/news groups. We're getting to better answers with much less work.
Instructors can create a Piazza for their class with a single click.
Students and instructors are automatically enrolled with appropriate roles, based on the course roster.
When clicking on Piazza in Blackboard, users are automatically signed in to Piazza, so there's no need to remember a separate user name and password.